diff --git a/src/lib/monitoring/types.ts b/src/lib/monitoring/types.ts
new file mode 100644
index 0000000..e8ee187
--- /dev/null
+++ b/src/lib/monitoring/types.ts
@@ -0,0 +1,148 @@
+// === System Health ===
+export interface SystemHealth {
+  cpuUsagePercent: number
+  memoryUsedMB: number
+  memoryTotalMB: number
+  memoryUsagePercent: number
+  diskUsedGB: number
+  diskTotalGB: number
+  diskUsagePercent: number
+  loadAvg1: number
+  loadAvg5: number
+  uptime: number // seconds
+}
+
+// === Service Statuses ===
+export type ServiceStatusType = 'online' | 'warning' | 'offline'
+
+export interface ProcessStatus {
+  status: ServiceStatusType
+  pid: number
+  memoryMB: number
+  uptimeSeconds: number
+  restarts: number
+}
+
+export interface PostgresqlStatus {
+  status: ServiceStatusType
+  connections: number
+  maxConnections: number
+  latencyMs: number
+}
+
+export interface PgBouncerStatus {
+  status: ServiceStatusType
+  activeConnections: number
+  waitingClients: number
+  poolSize: number
+}
+
+export interface RedisStatus {
+  status: ServiceStatusType
+  memoryUsedMB: number
+  connectedClients: number
+  opsPerSec: number
+}
+
+export interface ServiceStatuses {
+  payload: ProcessStatus
+  queueWorker: ProcessStatus
+  postgresql: PostgresqlStatus
+  pgbouncer: PgBouncerStatus
+  redis: RedisStatus
+}
+
+// === External Statuses ===
+export interface SmtpStatus {
+  status: ServiceStatusType
+  lastCheck: string // ISO date
+  responseTimeMs: number
+}
+
+export type OAuthStatusType = 'ok' | 'expiring_soon' | 'expired' | 'error'
+
+export interface OAuthTokenStatus {
+  status: OAuthStatusType
+  tokensTotal: number
+  tokensExpiringSoon: number
+  tokensExpired: number
+}
+
+export interface CronJobStatus {
+  lastRun: string // ISO date
+  status: 'ok' | 'failed' | 'unknown'
+}
+
+export interface CronStatuses {
+  communitySync: CronJobStatus
+  tokenRefresh: CronJobStatus
+  youtubeSync: CronJobStatus
+}
+
+export interface ExternalStatuses {
+  smtp: SmtpStatus
+  metaOAuth: OAuthTokenStatus
+  youtubeOAuth: OAuthTokenStatus
+  cronJobs: CronStatuses
+}
+
+// === Performance ===
+export interface PerformanceMetrics {
+  avgResponseTimeMs: number
+  p95ResponseTimeMs: number
+  p99ResponseTimeMs: number
+  errorRate: number // 0-1
+  requestsPerMinute: number
+}
+
+// === Full Snapshot ===
+export interface SystemMetrics {
+  timestamp: string // ISO date
+  system: SystemHealth
+  services: ServiceStatuses
+  external: ExternalStatuses
+  performance: PerformanceMetrics
+}
+
+// === SSE Events (discriminated union) ===
+export type MonitoringEvent =
+  | { type: 'health'; data: SystemHealth }
+  | { type: 'service'; data: Partial<ServiceStatuses> }
+  | { type: 'alert'; data: AlertEvent }
+  | { type: 'log'; data: LogEvent }
+  | { type: 'performance'; data: PerformanceMetrics }
+
+export interface AlertEvent {
+  id: string
+  ruleId: string
+  metric: string
+  value: number
+  threshold: number
+  severity: AlertSeverity
+  message: string
+  timestamp: string
+}
+
+export interface LogEvent {
+  id: string
+  level: LogLevel
+  source: LogSource
+  message: string
+  timestamp: string
+  context?: Record<string, unknown>
+}
+
+// === Enums as union types ===
+export type AlertCondition = 'gt' | 'lt' | 'eq' | 'gte' | 'lte'
+export type AlertSeverity = 'warning' | 'error' | 'critical'
+export type LogLevel = 'debug' | 'info' | 'warn' | 'error' | 'fatal'
+export type LogSource = 'payload' | 'queue-worker' | 'cron' | 'email' | 'oauth' | 'sync'
+
+// === Performance Tracker Entry ===
+export interface PerformanceEntry {
+  timestamp: number // Date.now()
+  method: string
+  path: string
+  statusCode: number
+  durationMs: number
+}
diff --git a/tests/unit/monitoring/types.test.ts b/tests/unit/monitoring/types.test.ts
new file mode 100644
index 0000000..fabb04d
--- /dev/null
+++ b/tests/unit/monitoring/types.test.ts
@@ -0,0 +1,93 @@
+import { describe, it, expect } from 'vitest'
+import type {
+  SystemMetrics,
+  MonitoringEvent,
+  AlertCondition,
+  LogLevel,
+  LogSource,
+} from '@/lib/monitoring/types'
+
+describe('Monitoring Types', () => {
+  it('SystemMetrics has all required sections', () => {
+    const metrics: SystemMetrics = {
+      timestamp: new Date().toISOString(),
+      system: {
+        cpuUsagePercent: 23,
+        memoryUsedMB: 4200,
+        memoryTotalMB: 8192,
+        memoryUsagePercent: 51.3,
+        diskUsedGB: 30,
+        diskTotalGB: 50,
+        diskUsagePercent: 60,
+        loadAvg1: 0.5,
+        loadAvg5: 0.8,
+        uptime: 1209600,
+      },
+      services: {
+        payload: {
+          status: 'online',
+          pid: 1234,
+          memoryMB: 512,
+          uptimeSeconds: 86400,
+          restarts: 0,
+        },
+        queueWorker: {
+          status: 'online',
+          pid: 5678,
+          memoryMB: 256,
+          uptimeSeconds: 86400,
+          restarts: 0,
+        },
+        postgresql: { status: 'online', connections: 12, maxConnections: 50, latencyMs: 2 },
+        pgbouncer: { status: 'online', activeConnections: 8, waitingClients: 0, poolSize: 20 },
+        redis: { status: 'online', memoryUsedMB: 48, connectedClients: 5, opsPerSec: 120 },
+      },
+      external: {
+        smtp: {
+          status: 'online',
+          lastCheck: new Date().toISOString(),
+          responseTimeMs: 180,
+        },
+        metaOAuth: { status: 'ok', tokensTotal: 2, tokensExpiringSoon: 1, tokensExpired: 0 },
+        youtubeOAuth: { status: 'ok', tokensTotal: 3, tokensExpiringSoon: 0, tokensExpired: 0 },
+        cronJobs: {
+          communitySync: { lastRun: new Date().toISOString(), status: 'ok' },
+          tokenRefresh: { lastRun: new Date().toISOString(), status: 'ok' },
+          youtubeSync: { lastRun: new Date().toISOString(), status: 'ok' },
+        },
+      },
+      performance: {
+        avgResponseTimeMs: 120,
+        p95ResponseTimeMs: 350,
+        p99ResponseTimeMs: 800,
+        errorRate: 0.02,
+        requestsPerMinute: 45,
+      },
+    }
+
+    expect(metrics.system.cpuUsagePercent).toBe(23)
+    expect(metrics.services.payload.status).toBe('online')
+    expect(metrics.external.smtp.status).toBe('online')
+    expect(metrics.performance.avgResponseTimeMs).toBe(120)
+  })
+
+  it('MonitoringEvent types are exhaustive', () => {
+    const events: MonitoringEvent['type'][] = ['health', 'service', 'alert', 'log', 'performance']
+    expect(events).toHaveLength(5)
+  })
+
+  it('AlertCondition covers all comparison operators', () => {
+    const conditions: AlertCondition[] = ['gt', 'lt', 'eq', 'gte', 'lte']
+    expect(conditions).toHaveLength(5)
+  })
+
+  it('LogLevel covers all severity levels', () => {
+    const levels: LogLevel[] = ['debug', 'info', 'warn', 'error', 'fatal']
+    expect(levels).toHaveLength(5)
+  })
+
+  it('LogSource covers all system components', () => {
+    const sources: LogSource[] = ['payload', 'queue-worker', 'cron', 'email', 'oauth', 'sync']
+    expect(sources).toHaveLength(6)
+  })
+})