diff --git a/backend/app/api/__init__.py b/backend/app/api/__init__.py
index ba6be4c..64a0825 100644
--- a/backend/app/api/__init__.py
+++ b/backend/app/api/__init__.py
@@ -38,6 +38,7 @@ class DeviceOut(BaseModel):
     first_seen: str | None  # ISO-8601 string
     last_seen: str | None
     ports: list[PortOut] = []
+    security_score: int  # 0–100; 100 = no risks
 
     model_config = {"from_attributes": True}
 
@@ -74,7 +75,11 @@ def list_devices(db: Annotated[Session, Depends(get_db)]) -> list[DeviceOut]:
     """Return all known devices with their open ports."""
     from app.models.device import Device
 
-    stmt = select(Device).options(selectinload(Device.ports)).order_by(Device.ip_address)
+    stmt = (
+        select(Device)
+        .options(selectinload(Device.ports), selectinload(Device.risks))
+        .order_by(Device.ip_address)
+    )
     devices = db.execute(stmt).scalars().all()
     return [_device_to_out(d) for d in devices]
 
@@ -84,7 +89,11 @@ def get_device(device_id: int, db: Annotated[Session, Depends(get_db)]) -> Devic
     """Return a single device by ID, including its ports."""
     from app.models.device import Device
 
-    stmt = select(Device).options(selectinload(Device.ports)).where(Device.id == device_id)
+    stmt = (
+        select(Device)
+        .options(selectinload(Device.ports), selectinload(Device.risks))
+        .where(Device.id == device_id)
+    )
     device = db.execute(stmt).scalar_one_or_none()
     if device is None:
         raise HTTPException(status_code=404, detail="Device not found")
@@ -104,7 +113,11 @@ def set_device_trusted(
     """Toggle the trusted flag on a device."""
     from app.models.device import Device
 
-    stmt = select(Device).options(selectinload(Device.ports)).where(Device.id == device_id)
+    stmt = (
+        select(Device)
+        .options(selectinload(Device.ports), selectinload(Device.risks))
+        .where(Device.id == device_id)
+    )
     device = db.execute(stmt).scalar_one_or_none()
     if device is None:
         raise HTTPException(status_code=404, detail="Device not found")
@@ -127,7 +140,11 @@ def set_device_label(
     """Set or clear the user-defined label on a device."""
     from app.models.device import Device
 
-    stmt = select(Device).options(selectinload(Device.ports)).where(Device.id == device_id)
+    stmt = (
+        select(Device)
+        .options(selectinload(Device.ports), selectinload(Device.risks))
+        .where(Device.id == device_id)
+    )
     device = db.execute(stmt).scalar_one_or_none()
     if device is None:
         raise HTTPException(status_code=404, detail="Device not found")
@@ -137,6 +154,15 @@ def set_device_label(
     return _device_to_out(device)
 
 
+def _device_security_score(d) -> int:  # noqa: ANN001 — SQLAlchemy instance
+    """Compute 0-100 security score from active risks. 100 = clean."""
+    if d.trusted:
+        return 100
+    weights = {"critical": 30, "high": 15, "medium": 7, "low": 3}
+    penalty = sum(weights.get(r.severity, 0) for r in d.risks)
+    return max(0, 100 - penalty)
+
+
 def _device_to_out(d) -> DeviceOut:  # noqa: ANN001 — SQLAlchemy instance, validated via Pydantic
     return DeviceOut(
         id=d.id,
@@ -149,6 +175,7 @@ def _device_to_out(d) -> DeviceOut:  # noqa: ANN001 — SQLAlchemy instance, val
         trusted=bool(d.trusted),
         first_seen=d.first_seen.isoformat() if d.first_seen else None,
         last_seen=d.last_seen.isoformat() if d.last_seen else None,
+        security_score=_device_security_score(d),
         ports=[
             PortOut(
                 id=p.id,
diff --git a/frontend/src/components/ScoreBadge.tsx b/frontend/src/components/ScoreBadge.tsx
new file mode 100644
index 0000000..b32d1ca
--- /dev/null
+++ b/frontend/src/components/ScoreBadge.tsx
@@ -0,0 +1,45 @@
+/**
+ * ScoreBadge — displays a device's 0–100 security score with colour coding.
+ * ≥70 green, 40–69 amber, <40 red.
+ */
+interface ScoreBadgeProps {
+  score: number;
+  size?: "sm" | "md" | "lg";
+  className?: string;
+}
+
+function scoreColour(score: number): string {
+  if (score >= 70)
+    return "text-[var(--color-accent-positive)] border-[var(--color-accent-positive)]/40 bg-[var(--color-accent-positive)]/10";
+  if (score >= 40)
+    return "text-[var(--color-accent-warning)] border-[var(--color-accent-warning)]/40 bg-[var(--color-accent-warning)]/10";
+  return "text-[var(--color-accent-danger)] border-[var(--color-accent-danger)]/40 bg-[var(--color-accent-danger)]/10";
+}
+
+const sizeClasses = {
+  sm: "text-xs px-1.5 py-0.5 min-w-[2.25rem]",
+  md: "text-sm px-2 py-1 min-w-[2.75rem]",
+  lg: "text-base px-3 py-1.5 min-w-[3.5rem] font-semibold",
+};
+
+export function ScoreBadge({
+  score,
+  size = "md",
+  className = "",
+}: ScoreBadgeProps) {
+  return (
+    <span
+      title={`Security score: ${score}/100`}
+      className={[
+        "inline-flex items-center justify-center rounded-full border font-mono font-medium",
+        sizeClasses[size],
+        scoreColour(score),
+        className,
+      ]
+        .filter(Boolean)
+        .join(" ")}
+    >
+      {score}
+    </span>
+  );
+}
diff --git a/frontend/src/components/index.ts b/frontend/src/components/index.ts
index deba70a..3cca15e 100644
--- a/frontend/src/components/index.ts
+++ b/frontend/src/components/index.ts
@@ -23,3 +23,5 @@ export { ScanBanner } from "./ScanBanner";
 export type { ScanBannerProps } from "./ScanBanner";
 
 export { PageHeader } from "./PageHeader";
+
+export { ScoreBadge } from "./ScoreBadge";
diff --git a/frontend/src/pages/DashboardPage.tsx b/frontend/src/pages/DashboardPage.tsx
index 885781c..e1f966c 100644
--- a/frontend/src/pages/DashboardPage.tsx
+++ b/frontend/src/pages/DashboardPage.tsx
@@ -11,6 +11,7 @@ import {
   ToastContainer,
   SkeletonCard,
   PageHeader,
+  ScoreBadge,
 } from "../components";
 import { useDevices, useScans, useTriggerScan, useRiskSummary } from "../hooks";
 import { useScanStatus } from "../hooks/useScanStatus";
@@ -236,7 +237,7 @@ export function DashboardPage() {
       ) : (
         <>
           {/* Summary stat cards */}
-          <div className="mb-6 grid gap-4 sm:grid-cols-2 lg:grid-cols-4">
+          <div className="mb-6 grid gap-4 sm:grid-cols-2 lg:grid-cols-5">
             <StatCard
               label="Total Devices"
               value={devices.length}
@@ -272,6 +273,25 @@ export function DashboardPage() {
               icon={<ShieldIcon />}
               to="/risks"
             />
+            <StatCard
+              label="Avg Score"
+              value={
+                devices.length > 0 ? (
+                  <ScoreBadge
+                    score={Math.round(
+                      devices.reduce((s, d) => s + d.security_score, 0) /
+                        devices.length,
+                    )}
+                    size="md"
+                  />
+                ) : (
+                  "—"
+                )
+              }
+              accentColor={STRIPE.devices}
+              icon={<ShieldIcon />}
+              to="/devices"
+            />
           </div>
 
           {/* Last scan */}
diff --git a/frontend/src/pages/DeviceDetailPage.tsx b/frontend/src/pages/DeviceDetailPage.tsx
index f897cb6..8d94320 100644
--- a/frontend/src/pages/DeviceDetailPage.tsx
+++ b/frontend/src/pages/DeviceDetailPage.tsx
@@ -4,7 +4,13 @@
  */
 import { memo, useRef, useMemo, useState } from "react";
 import { Link, useParams } from "react-router-dom";
-import { Card, Badge, SkeletonCard, PageHeader } from "../components";
+import {
+  Card,
+  Badge,
+  SkeletonCard,
+  PageHeader,
+  ScoreBadge,
+} from "../components";
 import { SEV_LEVELS } from "../constants/severity";
 import { useDevice, useRisks, useDeviceRecommendations } from "../hooks";
 import type { Risk, Recommendation, Severity } from "../types/api";
@@ -292,6 +298,17 @@ export function DeviceDetailPage() {
               </dd>
             </div>
           ))}
+          <div className="flex flex-col">
+            <dt className="text-xs font-semibold uppercase tracking-wider text-[var(--color-text-secondary)]">
+              Security Score
+            </dt>
+            <dd className="mt-1">
+              <ScoreBadge score={device.security_score} size="lg" />
+              <span className="ml-2 text-xs text-[var(--color-text-secondary)]">
+                / 100
+              </span>
+            </dd>
+          </div>
         </dl>
       </Card>
 
diff --git a/frontend/src/pages/DevicesPage.tsx b/frontend/src/pages/DevicesPage.tsx
index 6a88a29..304537c 100644
--- a/frontend/src/pages/DevicesPage.tsx
+++ b/frontend/src/pages/DevicesPage.tsx
@@ -4,7 +4,13 @@
  */
 import { useMemo, useRef, useState } from "react";
 import { Link } from "react-router-dom";
-import { Card, Badge, SkeletonTable, PageHeader } from "../components";
+import {
+  Card,
+  Badge,
+  SkeletonTable,
+  PageHeader,
+  ScoreBadge,
+} from "../components";
 import { useDevices, useRisks } from "../hooks";
 import type { Device } from "../types/api";
 
@@ -97,6 +103,7 @@ type SortKey =
   | "os_guess"
   | "ports"
   | "risks"
+  | "score"
   | "last_seen";
 type SortDir = "asc" | "desc";
 
@@ -130,6 +137,10 @@ function sortDevices(
         av = riskCounts[a.id] ?? 0;
         bv = riskCounts[b.id] ?? 0;
         break;
+      case "score":
+        av = a.security_score;
+        bv = b.security_score;
+        break;
       case "last_seen":
         av = a.last_seen ?? "";
         bv = b.last_seen ?? "";
@@ -268,6 +279,7 @@ export function DevicesPage() {
                       ["os_guess", "OS"],
                       ["ports", "Ports"],
                       ["risks", "Risks"],
+                      ["score", "Score"],
                       ["last_seen", "Last Seen"],
                     ] as [SortKey, string][]
                   ).map(([key, label]) => (
@@ -345,6 +357,9 @@ export function DevicesPage() {
                         <Badge variant="neutral">0</Badge>
                       )}
                     </td>
+                    <td className="px-4 py-3">
+                      <ScoreBadge score={device.security_score} size="sm" />
+                    </td>
                     <td className="px-4 py-3 text-[var(--color-text-secondary)]">
                       {device.last_seen
                         ? new Date(device.last_seen).toLocaleString()
diff --git a/frontend/src/types/api.ts b/frontend/src/types/api.ts
index b5d520f..09d1b96 100644
--- a/frontend/src/types/api.ts
+++ b/frontend/src/types/api.ts
@@ -23,6 +23,7 @@ export interface Device {
   first_seen: string | null; // ISO-8601
   last_seen: string | null;
   ports: Port[];
+  security_score: number; // 0–100
 }
 
 export interface Scan {