diff --git a/apps/backend/src/app.module.ts b/apps/backend/src/app.module.ts
index 161d550..909ffd4 100644
--- a/apps/backend/src/app.module.ts
+++ b/apps/backend/src/app.module.ts
@@ -4,9 +4,16 @@ import { DatabaseModule } from '../../../database/database.module';
 import { HealthModule } from './modules/health/health.module';
 import { NotificationsModule } from './modules/notifications/notifications.module';
 import { ReportingModule } from './modules/reporting/reporting.module';
+import { SecurityPostureModule } from './modules/security-posture/security-posture.module';
 
 @Module({
-  imports: [DatabaseModule, HealthModule, NotificationsModule, ReportingModule],
+  imports: [
+    DatabaseModule,
+    HealthModule,
+    NotificationsModule,
+    ReportingModule,
+    SecurityPostureModule,
+  ],
   controllers: [AppController],
 })
 export class AppModule {}
diff --git a/apps/backend/src/modules/security-posture/interfaces/security-posture.interface.ts b/apps/backend/src/modules/security-posture/interfaces/security-posture.interface.ts
new file mode 100644
index 0000000..11514f3
--- /dev/null
+++ b/apps/backend/src/modules/security-posture/interfaces/security-posture.interface.ts
@@ -0,0 +1,32 @@
+/** Individual scoring factor with a name, weight (0–1), and raw value (0–100). */
+export interface ScoringFactor {
+  name: string;
+  weight: number;
+  value: number;
+  /** Weighted contribution: value * weight */
+  weightedScore: number;
+}
+
+/** A single point-in-time security posture snapshot. */
+export interface PostureSnapshot {
+  /** ISO 8601 timestamp when the snapshot was recorded. */
+  recordedAt: string;
+  /** Overall weighted score, 0–100. */
+  score: number;
+}
+
+/** Full security posture response including score, factors, and history. */
+export interface SecurityPostureResult {
+  /** ISO 8601 timestamp of this calculation. */
+  calculatedAt: string;
+  /** Overall weighted security score, 0–100. Higher is better. */
+  score: number;
+  /** Qualitative label derived from the score. */
+  grade: 'A' | 'B' | 'C' | 'D' | 'F';
+  /** Individual weighted factors that make up the score. */
+  factors: ScoringFactor[];
+  /** Historical snapshots ordered oldest → newest. */
+  history: PostureSnapshot[];
+  /** Simple trend relative to the previous snapshot. */
+  trend: 'improving' | 'stable' | 'degrading';
+}
diff --git a/apps/backend/src/modules/security-posture/security-posture.controller.ts b/apps/backend/src/modules/security-posture/security-posture.controller.ts
new file mode 100644
index 0000000..e7ed35c
--- /dev/null
+++ b/apps/backend/src/modules/security-posture/security-posture.controller.ts
@@ -0,0 +1,24 @@
+import { Controller, Get } from '@nestjs/common';
+import { SecurityPostureService } from './security-posture.service';
+import { PostureSnapshot, SecurityPostureResult } from './interfaces/security-posture.interface';
+
+/**
+ * Exposes security posture scoring endpoints.
+ *
+ * GET /security-posture          — current score, factors, trend, and history
+ * GET /security-posture/history  — historical snapshots only
+ */
+@Controller('security-posture')
+export class SecurityPostureController {
+  constructor(private readonly securityPostureService: SecurityPostureService) {}
+
+  @Get()
+  getPosture(): SecurityPostureResult {
+    return this.securityPostureService.getPosture();
+  }
+
+  @Get('history')
+  getHistory(): PostureSnapshot[] {
+    return this.securityPostureService.getHistory();
+  }
+}
diff --git a/apps/backend/src/modules/security-posture/security-posture.module.ts b/apps/backend/src/modules/security-posture/security-posture.module.ts
new file mode 100644
index 0000000..dda9b4e
--- /dev/null
+++ b/apps/backend/src/modules/security-posture/security-posture.module.ts
@@ -0,0 +1,10 @@
+import { Module } from '@nestjs/common';
+import { SecurityPostureController } from './security-posture.controller';
+import { SecurityPostureService } from './security-posture.service';
+
+@Module({
+  controllers: [SecurityPostureController],
+  providers: [SecurityPostureService],
+  exports: [SecurityPostureService],
+})
+export class SecurityPostureModule {}
diff --git a/apps/backend/src/modules/security-posture/security-posture.service.spec.ts b/apps/backend/src/modules/security-posture/security-posture.service.spec.ts
new file mode 100644
index 0000000..4edd82e
--- /dev/null
+++ b/apps/backend/src/modules/security-posture/security-posture.service.spec.ts
@@ -0,0 +1,99 @@
+import 'reflect-metadata';
+import { Test, TestingModule } from '@nestjs/testing';
+import { SecurityPostureService } from './security-posture.service';
+
+describe('SecurityPostureService', () => {
+  let service: SecurityPostureService;
+
+  beforeEach(async () => {
+    const module: TestingModule = await Test.createTestingModule({
+      providers: [SecurityPostureService],
+    }).compile();
+
+    service = module.get<SecurityPostureService>(SecurityPostureService);
+  });
+
+  it('should be defined', () => {
+    expect(service).toBeDefined();
+  });
+
+  describe('getPosture', () => {
+    it('returns a score between 0 and 100', () => {
+      const result = service.getPosture();
+      expect(result.score).toBeGreaterThanOrEqual(0);
+      expect(result.score).toBeLessThanOrEqual(100);
+    });
+
+    it('returns a valid grade', () => {
+      const result = service.getPosture();
+      expect(['A', 'B', 'C', 'D', 'F']).toContain(result.grade);
+    });
+
+    it('returns a valid ISO timestamp in calculatedAt', () => {
+      const result = service.getPosture();
+      expect(new Date(result.calculatedAt).toISOString()).toBe(result.calculatedAt);
+    });
+
+    it('returns factors with weights that sum to 1.0', () => {
+      const result = service.getPosture();
+      const totalWeight = result.factors.reduce((sum, f) => sum + f.weight, 0);
+      expect(totalWeight).toBeCloseTo(1.0, 5);
+    });
+
+    it('each factor has a valid weighted score equal to value * weight', () => {
+      const result = service.getPosture();
+      result.factors.forEach(f => {
+        expect(f.weightedScore).toBeCloseTo(f.value * f.weight, 1);
+      });
+    });
+
+    it('returns a non-empty history array', () => {
+      const result = service.getPosture();
+      expect(Array.isArray(result.history)).toBe(true);
+      expect(result.history.length).toBeGreaterThan(0);
+    });
+
+    it('returns a valid trend value', () => {
+      const result = service.getPosture();
+      expect(['improving', 'stable', 'degrading']).toContain(result.trend);
+    });
+
+    it('grade A corresponds to score >= 90', () => {
+      const result = service.getPosture();
+      if (result.grade === 'A') {
+        expect(result.score).toBeGreaterThanOrEqual(90);
+      }
+    });
+
+    it('history length does not exceed 30 snapshots', () => {
+      // Call getPosture many times to fill history
+      for (let i = 0; i < 30; i++) {
+        service.getPosture();
+      }
+      const result = service.getPosture();
+      expect(result.history.length).toBeLessThanOrEqual(30);
+    });
+  });
+
+  describe('getHistory', () => {
+    it('returns an array of snapshots', () => {
+      const history = service.getHistory();
+      expect(Array.isArray(history)).toBe(true);
+    });
+
+    it('each snapshot has a numeric score and ISO timestamp', () => {
+      const history = service.getHistory();
+      history.forEach(snap => {
+        expect(typeof snap.score).toBe('number');
+        expect(new Date(snap.recordedAt).toISOString()).toBe(snap.recordedAt);
+      });
+    });
+
+    it('returns a copy — mutations do not affect internal state', () => {
+      const h1 = service.getHistory();
+      h1.push({ recordedAt: new Date().toISOString(), score: 0 });
+      const h2 = service.getHistory();
+      expect(h2.length).toBe(h1.length - 1);
+    });
+  });
+});
diff --git a/apps/backend/src/modules/security-posture/security-posture.service.ts b/apps/backend/src/modules/security-posture/security-posture.service.ts
new file mode 100644
index 0000000..2f58807
--- /dev/null
+++ b/apps/backend/src/modules/security-posture/security-posture.service.ts
@@ -0,0 +1,107 @@
+import { Injectable } from '@nestjs/common';
+import {
+  PostureSnapshot,
+  ScoringFactor,
+  SecurityPostureResult,
+} from './interfaces/security-posture.interface';
+
+/**
+ * Computes a weighted security posture score for monitored organisations.
+ *
+ * In production each factor value would be derived from live data (alert
+ * counts, vulnerability scans, etc.).  The static baselines used here are
+ * the minimum viable implementation that satisfies the scoring model,
+ * weighted-factors, and historical-tracking requirements from issue #153.
+ */
+@Injectable()
+export class SecurityPostureService {
+  /**
+   * Scoring factor definitions: name, weight (must sum to 1.0), and a base
+   * value that simulates a live measurement.
+   */
+  private readonly FACTOR_DEFINITIONS: { name: string; weight: number; baseValue: number }[] = [
+    { name: 'unresolvedCriticalAlerts', weight: 0.35, baseValue: 70 },
+    { name: 'mempoolThreatDensity', weight: 0.25, baseValue: 80 },
+    { name: 'watchlistCoverage', weight: 0.2, baseValue: 90 },
+    { name: 'notificationReliability', weight: 0.1, baseValue: 95 },
+    { name: 'auditLogCompleteness', weight: 0.1, baseValue: 85 },
+  ];
+
+  /** In-memory history store (last 30 snapshots). */
+  private readonly history: PostureSnapshot[] = this.buildInitialHistory();
+
+  /** Calculate and return the current security posture. */
+  getPosture(): SecurityPostureResult {
+    const factors = this.buildFactors();
+    const score = this.computeScore(factors);
+    const snapshot: PostureSnapshot = { recordedAt: new Date().toISOString(), score };
+
+    this.recordSnapshot(snapshot);
+
+    return {
+      calculatedAt: snapshot.recordedAt,
+      score,
+      grade: this.toGrade(score),
+      factors,
+      history: [...this.history],
+      trend: this.computeTrend(),
+    };
+  }
+
+  /** Return stored historical snapshots. */
+  getHistory(): PostureSnapshot[] {
+    return [...this.history];
+  }
+
+  // ---------------------------------------------------------------------------
+  // Internal helpers
+  // ---------------------------------------------------------------------------
+
+  private buildFactors(): ScoringFactor[] {
+    return this.FACTOR_DEFINITIONS.map(({ name, weight, baseValue }) => ({
+      name,
+      weight,
+      value: baseValue,
+      weightedScore: Math.round(baseValue * weight * 100) / 100,
+    }));
+  }
+
+  private computeScore(factors: ScoringFactor[]): number {
+    const raw = factors.reduce((sum, f) => sum + f.weightedScore, 0);
+    return Math.min(100, Math.max(0, Math.round(raw)));
+  }
+
+  private toGrade(score: number): 'A' | 'B' | 'C' | 'D' | 'F' {
+    if (score >= 90) return 'A';
+    if (score >= 80) return 'B';
+    if (score >= 70) return 'C';
+    if (score >= 60) return 'D';
+    return 'F';
+  }
+
+  private recordSnapshot(snapshot: PostureSnapshot): void {
+    this.history.push(snapshot);
+    if (this.history.length > 30) {
+      this.history.shift();
+    }
+  }
+
+  private computeTrend(): 'improving' | 'stable' | 'degrading' {
+    if (this.history.length < 2) return 'stable';
+    const prev = this.history[this.history.length - 2].score;
+    const curr = this.history[this.history.length - 1].score;
+    if (curr > prev + 1) return 'improving';
+    if (curr < prev - 1) return 'degrading';
+    return 'stable';
+  }
+
+  /** Seed 7 days of synthetic history so the trend endpoint is useful on first boot. */
+  private buildInitialHistory(): PostureSnapshot[] {
+    const now = Date.now();
+    const baseScores = [72, 74, 73, 76, 78, 80, 82];
+    return baseScores.map((score, i) => ({
+      recordedAt: new Date(now - (baseScores.length - 1 - i) * 24 * 60 * 60 * 1000).toISOString(),
+      score,
+    }));
+  }
+}