diff --git a/packages/analytics-client/src/analytics-client.ts b/packages/analytics-client/src/analytics-client.ts
index edcf576..3cc44c3 100644
--- a/packages/analytics-client/src/analytics-client.ts
+++ b/packages/analytics-client/src/analytics-client.ts
@@ -36,6 +36,7 @@ export class AnalyticsClient {
       scrollTracking: { enabled: false },
       trackResizes: false,
       resizeDebounceMs: 1000,
+      writeKey: undefined,
       ...config,
     };
 
@@ -180,7 +181,10 @@ export class AnalyticsClient {
     try {
       await fetch(`${this.config.apiBaseUrl}/analytics/track/interaction`, {
         method: 'POST',
-        headers: { 'Content-Type': 'application/json' },
+        headers: {
+          'Content-Type': 'application/json',
+          ...(this.config.writeKey ? { 'X-Write-Key': this.config.writeKey } : {}),
+        },
         body: JSON.stringify({ events }),
       });
 
@@ -227,12 +231,13 @@ export class AnalyticsClient {
   }
 
   private async sendViewEvents(events: ViewEventData[]): Promise<void> {
+    const headers: Record<string, string> = { 'Content-Type': 'application/json' };
+    if (this.config.writeKey) headers['X-Write-Key'] = this.config.writeKey;
+
     for (const event of events) {
       await fetch(`${this.config.apiBaseUrl}/analytics/track/view`, {
         method: 'POST',
-        headers: {
-          'Content-Type': 'application/json',
-        },
+        headers,
         body: JSON.stringify(event),
       });
     }
@@ -241,12 +246,13 @@ export class AnalyticsClient {
   private async sendEngagementEvents(
     events: EngagementEventData[],
   ): Promise<void> {
+    const headers: Record<string, string> = { 'Content-Type': 'application/json' };
+    if (this.config.writeKey) headers['X-Write-Key'] = this.config.writeKey;
+
     for (const event of events) {
       await fetch(`${this.config.apiBaseUrl}/analytics/track/engagement`, {
         method: 'POST',
-        headers: {
-          'Content-Type': 'application/json',
-        },
+        headers,
         body: JSON.stringify(event),
       });
     }
diff --git a/packages/analytics-client/src/backend-client.ts b/packages/analytics-client/src/backend-client.ts
index 50b6def..2bda75f 100644
--- a/packages/analytics-client/src/backend-client.ts
+++ b/packages/analytics-client/src/backend-client.ts
@@ -6,6 +6,8 @@ import type {
 export interface BackendAnalyticsConfig {
   apiBaseUrl: string;
   appName: string;
+  /** Write key sent as X-Write-Key header for collector authentication. Required in production. */
+  writeKey?: string;
   enableDebugLogging?: boolean;
 }
 
@@ -19,6 +21,7 @@ export class BackendAnalyticsClient {
   constructor(config: BackendAnalyticsConfig) {
     this.config = {
       enableDebugLogging: false,
+      writeKey: undefined,
       ...config,
     };
   }
@@ -102,13 +105,13 @@ export class BackendAnalyticsClient {
 
   private async sendEvent(type: 'view' | 'engagement', data: ViewEventData | EngagementEventData): Promise<void> {
     const endpoint = type === 'view' ? '/analytics/track/view' : '/analytics/track/engagement';
+    const headers: Record<string, string> = { 'Content-Type': 'application/json' };
+    if (this.config.writeKey) headers['X-Write-Key'] = this.config.writeKey;
 
     try {
       const response = await fetch(`${this.config.apiBaseUrl}${endpoint}`, {
         method: 'POST',
-        headers: {
-          'Content-Type': 'application/json',
-        },
+        headers,
         body: JSON.stringify(data),
         signal: AbortSignal.timeout(5000), // 5 second timeout
       });
diff --git a/packages/analytics-client/src/types.ts b/packages/analytics-client/src/types.ts
index b6b7e7f..5927436 100644
--- a/packages/analytics-client/src/types.ts
+++ b/packages/analytics-client/src/types.ts
@@ -42,6 +42,8 @@ export interface ScrollTrackingConfig {
 export interface AnalyticsConfig {
   apiBaseUrl: string
   appName: string
+  /** Write key sent as X-Write-Key header for collector authentication. Required in production. */
+  writeKey?: string
   batchSize?: number
   batchInterval?: number
   enableDebugLogging?: boolean