diff --git a/.env.example b/.env.example
index e872649..c11d0a6 100644
--- a/.env.example
+++ b/.env.example
@@ -16,3 +16,13 @@ BACKEND_PORT=8000
 
 # AI Design Analysis (optional — leave empty to disable)
 ANTHROPIC_API_KEY=
+
+# Azure AD SSO (set AZURE_AUTH_ENABLED=false to disable)
+AZURE_AUTH_ENABLED=true
+AZURE_TENANT_ID=e519c2e6-bc6d-4fdf-8d9c-923c2f002385
+AZURE_CLIENT_ID=9079054c-9620-4757-a256-23413042f1ef
+
+# Frontend Azure AD (Vite env vars)
+VITE_AZURE_TENANT_ID=e519c2e6-bc6d-4fdf-8d9c-923c2f002385
+VITE_AZURE_CLIENT_ID=9079054c-9620-4757-a256-23413042f1ef
+VITE_AZURE_REDIRECT_URI=https://ai-sandbox.oliver.solutions/olivas
diff --git a/backend/app/api/endpoints/analysis.py b/backend/app/api/endpoints/analysis.py
index 2bf23d7..7f76fc0 100644
--- a/backend/app/api/endpoints/analysis.py
+++ b/backend/app/api/endpoints/analysis.py
@@ -1,6 +1,6 @@
 import io
 
-from fastapi import APIRouter, BackgroundTasks, Depends, Header, HTTPException, UploadFile, Form
+from fastapi import APIRouter, BackgroundTasks, Depends, HTTPException, UploadFile, Form
 from fastapi.responses import StreamingResponse
 from PIL import Image
 from sqlalchemy import select
@@ -27,10 +27,8 @@ async def create_analysis(
     name: str | None = Form(None),
     model: str = Form("deepgaze_iie"),
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
-
     # Verify project belongs to user
     stmt = select(Project).where(Project.id == project_id, Project.user_id == user_id)
     result = await db.execute(stmt)
@@ -474,9 +472,8 @@ async def check_ai_insights_available():
 async def get_analysis(
     analysis_id: str,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     stmt = select(Analysis).where(Analysis.id == analysis_id, Analysis.user_id == user_id)
     result = await db.execute(stmt)
     analysis = result.scalar_one_or_none()
@@ -515,9 +512,8 @@ async def get_analysis(
 async def get_analysis_status(
     analysis_id: str,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     stmt = select(Analysis).where(Analysis.id == analysis_id, Analysis.user_id == user_id)
     result = await db.execute(stmt)
     analysis = result.scalar_one_or_none()
@@ -531,9 +527,8 @@ async def get_analysis_image(
     analysis_id: str,
     image_type: str,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     stmt = select(Analysis).where(Analysis.id == analysis_id, Analysis.user_id == user_id)
     result = await db.execute(stmt)
     analysis = result.scalar_one_or_none()
@@ -564,7 +559,7 @@ async def get_analysis_image(
 async def generate_ai_insights_endpoint(
     analysis_id: str,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
     """Generate AI-powered insights for a completed analysis using Claude."""
     from app.services.ai_insights import generate_ai_insights, is_available
@@ -572,7 +567,6 @@ async def generate_ai_insights_endpoint(
     if not is_available():
         raise HTTPException(status_code=503, detail="AI insights not configured (missing API key)")
 
-    user_id = get_user_id(x_user_id)
     stmt = select(Analysis).where(Analysis.id == analysis_id, Analysis.user_id == user_id)
     result = await db.execute(stmt)
     analysis = result.scalar_one_or_none()
@@ -632,9 +626,8 @@ async def generate_ai_insights_endpoint(
 async def delete_analysis(
     analysis_id: str,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     stmt = select(Analysis).where(Analysis.id == analysis_id, Analysis.user_id == user_id)
     result = await db.execute(stmt)
     analysis = result.scalar_one_or_none()
diff --git a/backend/app/api/endpoints/aoi.py b/backend/app/api/endpoints/aoi.py
index a7b598a..b8307a6 100644
--- a/backend/app/api/endpoints/aoi.py
+++ b/backend/app/api/endpoints/aoi.py
@@ -1,5 +1,5 @@
 import numpy as np
-from fastapi import APIRouter, Depends, Header, HTTPException
+from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -19,9 +19,8 @@ async def create_aois(
     analysis_id: str,
     body: AOICreate,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     stmt = select(Analysis).where(Analysis.id == analysis_id, Analysis.user_id == user_id)
     result = await db.execute(stmt)
     analysis = result.scalar_one_or_none()
@@ -68,9 +67,8 @@ async def create_aois(
 async def list_aois(
     analysis_id: str,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     stmt = select(Analysis).where(Analysis.id == analysis_id, Analysis.user_id == user_id)
     result = await db.execute(stmt)
     analysis = result.scalar_one_or_none()
@@ -88,9 +86,8 @@ async def delete_aoi(
     analysis_id: str,
     aoi_id: str,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     # Verify analysis ownership
     stmt = select(Analysis).where(Analysis.id == analysis_id, Analysis.user_id == user_id)
     result = await db.execute(stmt)
diff --git a/backend/app/api/endpoints/comparison.py b/backend/app/api/endpoints/comparison.py
index 94be9ef..70836a8 100644
--- a/backend/app/api/endpoints/comparison.py
+++ b/backend/app/api/endpoints/comparison.py
@@ -1,4 +1,5 @@
-from fastapi import APIRouter, Depends, Header, HTTPException
+from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -11,13 +12,6 @@ from app.models.project import Project
 router = APIRouter(tags=["comparison"])
 
 
-class ComparisonCreate:
-    pass
-
-
-from pydantic import BaseModel
-
-
 class ComparisonCreateBody(BaseModel):
     name: str
     analysis_ids: list[str]
@@ -39,10 +33,8 @@ async def create_comparison(
     project_id: str,
     body: ComparisonCreateBody,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
-
     # Verify project
     stmt = select(Project).where(Project.id == project_id, Project.user_id == user_id)
     result = await db.execute(stmt)
@@ -111,9 +103,8 @@ async def create_comparison(
 async def get_comparison(
     comparison_id: str,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     stmt = select(Comparison).where(
         Comparison.id == comparison_id, Comparison.user_id == user_id
     )
diff --git a/backend/app/api/endpoints/projects.py b/backend/app/api/endpoints/projects.py
index a4fc915..f583db6 100644
--- a/backend/app/api/endpoints/projects.py
+++ b/backend/app/api/endpoints/projects.py
@@ -1,4 +1,4 @@
-from fastapi import APIRouter, Depends, Header, HTTPException
+from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload
@@ -16,9 +16,8 @@ router = APIRouter(prefix="/projects", tags=["projects"])
 async def create_project(
     body: ProjectCreate,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     project = Project(user_id=user_id, name=body.name, description=body.description)
     db.add(project)
     await db.flush()
@@ -38,9 +37,8 @@ async def list_projects(
     page: int = 1,
     per_page: int = 20,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     offset = (page - 1) * per_page
 
     stmt = (
@@ -75,9 +73,8 @@ async def list_projects(
 async def get_project(
     project_id: str,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     stmt = (
         select(Project)
         .options(selectinload(Project.analyses))
@@ -95,9 +92,8 @@ async def update_project(
     project_id: str,
     body: ProjectUpdate,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     stmt = select(Project).where(Project.id == project_id, Project.user_id == user_id)
     result = await db.execute(stmt)
     project = result.scalar_one_or_none()
@@ -130,9 +126,8 @@ async def update_project(
 async def delete_project(
     project_id: str,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     stmt = select(Project).where(Project.id == project_id, Project.user_id == user_id)
     result = await db.execute(stmt)
     project = result.scalar_one_or_none()
diff --git a/backend/app/api/endpoints/reports.py b/backend/app/api/endpoints/reports.py
index ecf8b10..08f0368 100644
--- a/backend/app/api/endpoints/reports.py
+++ b/backend/app/api/endpoints/reports.py
@@ -1,7 +1,7 @@
 import io
 import logging
 
-from fastapi import APIRouter, Depends, Header, HTTPException
+from fastapi import APIRouter, Depends, HTTPException
 from fastapi.responses import StreamingResponse
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -20,9 +20,8 @@ logger = logging.getLogger("olivas.reports")
 async def download_report(
     analysis_id: str,
     db: AsyncSession = Depends(get_db),
-    x_user_id: str | None = Header(None),
+    user_id: str = Depends(get_user_id),
 ):
-    user_id = get_user_id(x_user_id)
     stmt = (
         select(Analysis)
         .options(selectinload(Analysis.aois))
diff --git a/backend/app/auth.py b/backend/app/auth.py
new file mode 100644
index 0000000..7ffaa97
--- /dev/null
+++ b/backend/app/auth.py
@@ -0,0 +1,97 @@
+"""Azure AD JWT validation using JWKS (RS256)."""
+
+import logging
+import time
+from dataclasses import dataclass
+
+import httpx
+import jwt
+
+from app.config import settings
+
+logger = logging.getLogger("olivas.auth")
+
+# JWKS cache
+_jwks_cache: dict = {}
+_jwks_cache_time: float = 0
+_JWKS_CACHE_TTL = 3600  # 1 hour
+
+
+@dataclass
+class CurrentUser:
+    oid: str
+    name: str
+    email: str
+
+
+def _get_jwks_uri() -> str:
+    tenant = settings.AZURE_TENANT_ID
+    return f"https://login.microsoftonline.com/{tenant}/discovery/v2.0/keys"
+
+
+def _get_issuer() -> str:
+    tenant = settings.AZURE_TENANT_ID
+    return f"https://login.microsoftonline.com/{tenant}/v2.0"
+
+
+def refresh_jwks_cache() -> None:
+    """Fetch and cache JWKS keys from Azure AD."""
+    global _jwks_cache, _jwks_cache_time
+    uri = _get_jwks_uri()
+    logger.info(f"Fetching JWKS from {uri}")
+    resp = httpx.get(uri, timeout=10)
+    resp.raise_for_status()
+    _jwks_cache = resp.json()
+    _jwks_cache_time = time.time()
+    logger.info(f"JWKS cache refreshed ({len(_jwks_cache.get('keys', []))} keys)")
+
+
+def _get_signing_key(token: str) -> jwt.algorithms.RSAAlgorithm:
+    """Get the signing key for the given token from cached JWKS."""
+    global _jwks_cache, _jwks_cache_time
+
+    if not _jwks_cache or (time.time() - _jwks_cache_time > _JWKS_CACHE_TTL):
+        refresh_jwks_cache()
+
+    jwks_client = jwt.PyJWKClient.__new__(jwt.PyJWKClient)
+    jwks_client.jwk_set = jwt.PyJWKSet.from_dict(_jwks_cache)
+
+    try:
+        header = jwt.get_unverified_header(token)
+        kid = header.get("kid")
+        for key in jwks_client.jwk_set.keys:
+            if key.key_id == kid:
+                return key.key
+    except Exception:
+        pass
+
+    # Key not found — try refreshing once
+    refresh_jwks_cache()
+    jwks_client.jwk_set = jwt.PyJWKSet.from_dict(_jwks_cache)
+    header = jwt.get_unverified_header(token)
+    kid = header.get("kid")
+    for key in jwks_client.jwk_set.keys:
+        if key.key_id == kid:
+            return key.key
+
+    raise jwt.InvalidTokenError(f"Unable to find signing key with kid={kid}")
+
+
+def validate_token(token: str) -> CurrentUser:
+    """Decode and validate an Azure AD JWT token. Returns CurrentUser."""
+    signing_key = _get_signing_key(token)
+
+    payload = jwt.decode(
+        token,
+        signing_key,
+        algorithms=["RS256"],
+        audience=settings.AZURE_CLIENT_ID,
+        issuer=_get_issuer(),
+        options={"require": ["exp", "iss", "aud", "oid"]},
+    )
+
+    return CurrentUser(
+        oid=payload["oid"],
+        name=payload.get("name", ""),
+        email=payload.get("preferred_username", payload.get("email", "")),
+    )
diff --git a/backend/app/config.py b/backend/app/config.py
index 6b2e250..2c78cf3 100644
--- a/backend/app/config.py
+++ b/backend/app/config.py
@@ -17,6 +17,11 @@ class Settings(BaseSettings):
 
     GOOGLE_CLOUD_PROJECT: str = "optical-414516"
 
+    # Azure AD SSO
+    AZURE_TENANT_ID: str = ""
+    AZURE_CLIENT_ID: str = ""
+    AZURE_AUTH_ENABLED: bool = True
+
     @property
     def use_cloud_run(self) -> bool:
         return bool(self.CLOUD_RUN_SALIENCY_URL)
diff --git a/backend/app/dependencies.py b/backend/app/dependencies.py
index b4b00ce..e4f8086 100644
--- a/backend/app/dependencies.py
+++ b/backend/app/dependencies.py
@@ -1,12 +1,36 @@
-from sqlalchemy.ext.asyncio import AsyncSession
+import logging
 
+from fastapi import Depends, Header, HTTPException
+
+from app.auth import CurrentUser, validate_token
+from app.config import settings
 from app.db.session import get_db
 
-# User ID header — placeholder for SSO integration.
-# When SSO is added, this will extract user_id from the JWT/session token.
-USER_ID_HEADER = "X-User-Id"
-DEFAULT_USER_ID = "default"
+logger = logging.getLogger("olivas.auth")
+
+_anonymous_user = CurrentUser(oid="default", name="Default User", email="")
 
 
-def get_user_id(x_user_id: str | None = None) -> str:
-    return x_user_id or DEFAULT_USER_ID
+async def get_current_user(authorization: str | None = Header(None)) -> CurrentUser:
+    """Extract and validate the Bearer token from Authorization header."""
+    if not settings.AZURE_AUTH_ENABLED:
+        return _anonymous_user
+
+    if not authorization:
+        raise HTTPException(status_code=401, detail="Missing Authorization header")
+
+    parts = authorization.split(" ", 1)
+    if len(parts) != 2 or parts[0].lower() != "bearer":
+        raise HTTPException(status_code=401, detail="Invalid Authorization header format")
+
+    token = parts[1]
+    try:
+        return validate_token(token)
+    except Exception as e:
+        logger.warning(f"Token validation failed: {e}")
+        raise HTTPException(status_code=401, detail="Invalid or expired token")
+
+
+async def get_user_id(user: CurrentUser = Depends(get_current_user)) -> str:
+    """Return the user's OID from the validated token."""
+    return user.oid
diff --git a/backend/app/main.py b/backend/app/main.py
index 3edb837..31f79f8 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -35,6 +35,15 @@ async def lifespan(app: FastAPI):
     except Exception as e:
         logger.warning(f"Failed to load ML models: {e}. Analysis will fail until models load.")
 
+    # Warm up JWKS cache for Azure AD token validation
+    if settings.AZURE_AUTH_ENABLED and settings.AZURE_TENANT_ID:
+        try:
+            from app.auth import refresh_jwks_cache
+            refresh_jwks_cache()
+            logger.info("Azure AD JWKS cache warmed up")
+        except Exception as e:
+            logger.warning(f"Failed to warm JWKS cache: {e}. Will fetch on first request.")
+
     yield
 
     # Shutdown
diff --git a/backend/pyproject.toml b/backend/pyproject.toml
index 9c4e383..77e2449 100644
--- a/backend/pyproject.toml
+++ b/backend/pyproject.toml
@@ -21,6 +21,7 @@ dependencies = [
     "aiofiles>=23.0",
     "anthropic>=0.40",
     "httpx>=0.27",
+    "PyJWT[crypto]>=2.8",
 ]
 
 [project.optional-dependencies]
diff --git a/docker-compose.yml b/docker-compose.yml
index 6f476dd..9128279 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -30,6 +30,10 @@ services:
       CLOUD_RUN_PROCESSING_URL: ${CLOUD_RUN_PROCESSING_URL:-}
       CLOUD_RUN_SECRET: ${CLOUD_RUN_SECRET:-}
       GOOGLE_CLOUD_PROJECT: ${GOOGLE_CLOUD_PROJECT:-optical-414516}
+      # Azure AD SSO
+      AZURE_AUTH_ENABLED: ${AZURE_AUTH_ENABLED:-true}
+      AZURE_TENANT_ID: ${AZURE_TENANT_ID:-}
+      AZURE_CLIENT_ID: ${AZURE_CLIENT_ID:-}
     volumes:
       - uploads:/app/data/uploads
     depends_on:
diff --git a/frontend/package-lock.json b/frontend/package-lock.json
index 2502453..a0e0df7 100644
--- a/frontend/package-lock.json
+++ b/frontend/package-lock.json
@@ -8,6 +8,8 @@
       "name": "frontend",
       "version": "0.0.0",
       "dependencies": {
+        "@azure/msal-browser": "^5.4.0",
+        "@azure/msal-react": "^5.0.6",
         "@tailwindcss/vite": "^4.2.1",
         "@tanstack/react-query": "^5.90.21",
         "axios": "^1.13.5",
@@ -35,6 +37,40 @@
         "vite": "^7.3.1"
       }
     },
+    "node_modules/@azure/msal-browser": {
+      "version": "5.4.0",
+      "resolved": "https://registry.npmjs.org/@azure/msal-browser/-/msal-browser-5.4.0.tgz",
+      "integrity": "sha512-GvRbLNk26oPOPpnry4Ym8wtXrmdozGm2Sry5EKfui0siwnEuAKWEeMLLyosDo5nVEIIDO1C2t/+HpVzqqCWlfQ==",
+      "license": "MIT",
+      "dependencies": {
+        "@azure/msal-common": "16.2.0"
+      },
+      "engines": {
+        "node": ">=0.8.0"
+      }
+    },
+    "node_modules/@azure/msal-common": {
+      "version": "16.2.0",
+      "resolved": "https://registry.npmjs.org/@azure/msal-common/-/msal-common-16.2.0.tgz",
+      "integrity": "sha512-ge0nGzTLmEE5lg7tSCbTBrYqMGkpFQeQEtqfcKPuGJn/FPFf8Xz51uDfZsm5xpstNZGMYPhHvnYbL8OeNp/aLw==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=0.8.0"
+      }
+    },
+    "node_modules/@azure/msal-react": {
+      "version": "5.0.6",
+      "resolved": "https://registry.npmjs.org/@azure/msal-react/-/msal-react-5.0.6.tgz",
+      "integrity": "sha512-p0YTCdsx+jkZNR/3Awznn12LxlTcKdsH4/FBiIoD53axn6A4y8g+9sng716PtxMvXSGaRIZo9JKM/yfFdz/+oQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=20"
+      },
+      "peerDependencies": {
+        "@azure/msal-browser": "^5.4.0",
+        "react": "^19.2.1"
+      }
+    },
     "node_modules/@babel/code-frame": {
       "version": "7.29.0",
       "resolved": "https://registry.npmjs.org/@babel/code-frame/-/code-frame-7.29.0.tgz",
diff --git a/frontend/package.json b/frontend/package.json
index 1683c85..acb0283 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -10,6 +10,8 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "@azure/msal-browser": "^5.4.0",
+    "@azure/msal-react": "^5.0.6",
     "@tailwindcss/vite": "^4.2.1",
     "@tanstack/react-query": "^5.90.21",
     "axios": "^1.13.5",
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index e9e3108..1d730a9 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -1,5 +1,6 @@
 import { Routes, Route } from "react-router-dom";
 import AppLayout from "./components/layout/AppLayout";
+import RequireAuth from "./auth/RequireAuth";
 import Dashboard from "./pages/Dashboard";
 import ProjectDetail from "./pages/ProjectDetail";
 import NewAnalysis from "./pages/NewAnalysis";
@@ -10,17 +11,19 @@ import About from "./pages/About";
 
 function App() {
   return (
-    <Routes>
-      <Route element={<AppLayout />}>
-        <Route path="/" element={<Dashboard />} />
-        <Route path="/projects/:projectId" element={<ProjectDetail />} />
-        <Route path="/analyze" element={<NewAnalysis />} />
-        <Route path="/analyze/:analysisId" element={<AnalysisView />} />
-        <Route path="/compare/:comparisonId" element={<ComparisonView />} />
-        <Route path="/help" element={<Help />} />
-        <Route path="/about" element={<About />} />
-      </Route>
-    </Routes>
+    <RequireAuth>
+      <Routes>
+        <Route element={<AppLayout />}>
+          <Route path="/" element={<Dashboard />} />
+          <Route path="/projects/:projectId" element={<ProjectDetail />} />
+          <Route path="/analyze" element={<NewAnalysis />} />
+          <Route path="/analyze/:analysisId" element={<AnalysisView />} />
+          <Route path="/compare/:comparisonId" element={<ComparisonView />} />
+          <Route path="/help" element={<Help />} />
+          <Route path="/about" element={<About />} />
+        </Route>
+      </Routes>
+    </RequireAuth>
   );
 }
 
diff --git a/frontend/src/api/client.ts b/frontend/src/api/client.ts
index 902346c..8902e86 100644
--- a/frontend/src/api/client.ts
+++ b/frontend/src/api/client.ts
@@ -1,4 +1,5 @@
 import axios from "axios";
+import { msalInstance, loginRequest } from "../auth/msalConfig";
 
 const client = axios.create({
   baseURL: "/api",
@@ -7,14 +8,36 @@ const client = axios.create({
   },
 });
 
-client.interceptors.request.use((config) => {
-  config.headers["X-User-Id"] = "default";
+client.interceptors.request.use(async (config) => {
+  const accounts = msalInstance.getAllAccounts();
+  if (accounts.length > 0) {
+    try {
+      const response = await msalInstance.acquireTokenSilent({
+        ...loginRequest,
+        account: accounts[0],
+      });
+      config.headers.Authorization = `Bearer ${response.idToken}`;
+    } catch (e) {
+      console.warn("[API] Silent token acquisition failed, will redirect:", e);
+      await msalInstance.acquireTokenRedirect(loginRequest);
+    }
+  }
   return config;
 });
 
 client.interceptors.response.use(
   (response) => response,
-  (error) => {
+  async (error) => {
+    if (error.response?.status === 401) {
+      const accounts = msalInstance.getAllAccounts();
+      if (accounts.length > 0) {
+        try {
+          await msalInstance.acquireTokenRedirect(loginRequest);
+        } catch (e) {
+          console.error("[API] Re-auth redirect failed:", e);
+        }
+      }
+    }
     const message =
       error.response?.data?.detail || error.message || "An error occurred";
     console.error("[API Error]", message);
diff --git a/frontend/src/auth/RequireAuth.tsx b/frontend/src/auth/RequireAuth.tsx
new file mode 100644
index 0000000..6fe1402
--- /dev/null
+++ b/frontend/src/auth/RequireAuth.tsx
@@ -0,0 +1,36 @@
+import { useEffect } from "react";
+import { useIsAuthenticated, useMsal } from "@azure/msal-react";
+import { InteractionStatus } from "@azure/msal-browser";
+import { loginRequest } from "./msalConfig";
+import LoadingSpinner from "../components/common/LoadingSpinner";
+
+export default function RequireAuth({ children }: { children: React.ReactNode }) {
+  const isAuthenticated = useIsAuthenticated();
+  const { inProgress, instance } = useMsal();
+
+  useEffect(() => {
+    if (!isAuthenticated && inProgress === InteractionStatus.None) {
+      instance.loginRedirect(loginRequest).catch((e) => {
+        console.error("[Auth] Login redirect failed:", e);
+      });
+    }
+  }, [isAuthenticated, inProgress, instance]);
+
+  if (inProgress !== InteractionStatus.None) {
+    return (
+      <div className="flex items-center justify-center min-h-screen">
+        <LoadingSpinner size="lg" message="Authenticating..." />
+      </div>
+    );
+  }
+
+  if (!isAuthenticated) {
+    return (
+      <div className="flex items-center justify-center min-h-screen">
+        <LoadingSpinner size="lg" message="Redirecting to login..." />
+      </div>
+    );
+  }
+
+  return <>{children}</>;
+}
diff --git a/frontend/src/auth/msalConfig.ts b/frontend/src/auth/msalConfig.ts
new file mode 100644
index 0000000..af777c5
--- /dev/null
+++ b/frontend/src/auth/msalConfig.ts
@@ -0,0 +1,31 @@
+import { PublicClientApplication, type Configuration, LogLevel } from "@azure/msal-browser";
+
+const clientId = import.meta.env.VITE_AZURE_CLIENT_ID ?? "";
+const tenantId = import.meta.env.VITE_AZURE_TENANT_ID ?? "";
+const redirectUri = import.meta.env.VITE_AZURE_REDIRECT_URI ?? window.location.origin;
+
+const msalConfig: Configuration = {
+  auth: {
+    clientId,
+    authority: `https://login.microsoftonline.com/${tenantId}`,
+    redirectUri,
+    postLogoutRedirectUri: redirectUri,
+  },
+  cache: {
+    cacheLocation: "sessionStorage",
+  },
+  system: {
+    loggerOptions: {
+      logLevel: LogLevel.Warning,
+      loggerCallback: (_level, message) => {
+        console.debug("[MSAL]", message);
+      },
+    },
+  },
+};
+
+export const msalInstance = new PublicClientApplication(msalConfig);
+
+export const loginRequest = {
+  scopes: [`${clientId}/.default`],
+};
diff --git a/frontend/src/components/layout/Header.tsx b/frontend/src/components/layout/Header.tsx
index 813f3e2..6d69b50 100644
--- a/frontend/src/components/layout/Header.tsx
+++ b/frontend/src/components/layout/Header.tsx
@@ -1,8 +1,16 @@
 import { Link } from "react-router-dom";
+import { useMsal } from "@azure/msal-react";
 import { useUIStore } from "../../stores/uiStore";
 
 export default function Header() {
   const toggleSidebar = useUIStore((s) => s.toggleSidebar);
+  const { instance, accounts } = useMsal();
+
+  const userName = accounts[0]?.name || accounts[0]?.username || "";
+
+  const handleLogout = () => {
+    instance.logoutRedirect({ postLogoutRedirectUri: window.location.origin });
+  };
 
   return (
     <header
@@ -71,6 +79,19 @@ export default function Header() {
         >
           New Analysis
         </Link>
+        {userName && (
+          <span className="text-sm text-white/60 ml-2 hidden md:inline">
+            {userName}
+          </span>
+        )}
+        {accounts.length > 0 && (
+          <button
+            onClick={handleLogout}
+            className="text-sm text-white/50 hover:text-white transition-colors px-2 py-1.5 rounded"
+          >
+            Logout
+          </button>
+        )}
       </nav>
     </header>
   );
diff --git a/frontend/src/main.tsx b/frontend/src/main.tsx
index a601f65..41ab315 100644
--- a/frontend/src/main.tsx
+++ b/frontend/src/main.tsx
@@ -2,6 +2,8 @@ import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
 import { BrowserRouter } from "react-router-dom";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { MsalProvider } from "@azure/msal-react";
+import { msalInstance } from "./auth/msalConfig";
 import App from "./App";
 import "./globals.css";
 
@@ -15,12 +17,21 @@ const queryClient = new QueryClient({
   },
 });
 
-createRoot(document.getElementById("root")!).render(
-  <StrictMode>
-    <QueryClientProvider client={queryClient}>
-      <BrowserRouter>
-        <App />
-      </BrowserRouter>
-    </QueryClientProvider>
-  </StrictMode>,
-);
+// Initialize MSAL — handle redirect promise
+msalInstance.initialize().then(() => {
+  msalInstance.handleRedirectPromise().catch((e) => {
+    console.error("[MSAL] Redirect error:", e);
+  });
+
+  createRoot(document.getElementById("root")!).render(
+    <StrictMode>
+      <MsalProvider instance={msalInstance}>
+        <QueryClientProvider client={queryClient}>
+          <BrowserRouter>
+            <App />
+          </BrowserRouter>
+        </QueryClientProvider>
+      </MsalProvider>
+    </StrictMode>,
+  );
+});