diff --git a/lib/Analytics.js b/lib/Analytics.js
index 8541ba4..fc034d1 100644
--- a/lib/Analytics.js
+++ b/lib/Analytics.js
@@ -6,6 +6,7 @@ export default class Analytics {
     this.trackingId = trackingId;
     this.clientId = clientId;
     this.userAgent = userAgent || null;
+    this.userId = null;
     this.customDimensions = new CustomDimensions();
 
     if (!userAgent) {
@@ -15,18 +16,24 @@ export default class Analytics {
 
   addDimension(index, name) {
     this.customDimensions.addDimension(index, name);
+
   }
 
   removeDimension(index) {
     this.customDimensions.removeDimension(index);
   }
 
+  setUserId(userId) {
+    this.userId = userId;
+  }
+
   send(hit) {
 
+    let uidstr = '';
     hit.set({
       v: this.version,
       tid: this.trackingId,
-      cid: this.clientId
+      cid: this.clientId,
     });
 
     let options = {
@@ -36,6 +43,9 @@ export default class Analytics {
       }
     }
 
-    return fetch(`https://www.google-analytics.com/collect?${hit.toQueryString()}&${this.customDimensions.toQueryString()}&z=${Math.round(Math.random() * 1e8)}`, options);
+    if (this.userId !== null && this.userId !== undefined) {
+      uidstr = `uid=${this.userId}&`;
+    }
+    return fetch(`https://www.google-analytics.com/collect?${uidstr}${hit.toQueryString()}&${this.customDimensions.toQueryString()}&z=${Math.round(Math.random() * 1e8)}`, options);
   }
 }