Open Google Analytics 4, click the gear icon for Admin in the bottom left, then under the Property column select Data Streams. Click your web data stream (usually your domain name), and you'll see the Measurement ID displayed prominently at the top right of the stream details screen. It follows the format G-XXXXXXXXXX where the Xs are alphanumeric characters. This Measurement ID replaces the old Universal Analytics tracking ID (UA-XXXXXXXXX-X format). You need this ID to configure Google Tag Manager, add the gtag.js snippet directly to your site header, or set up tracking in WordPress plugins like Site Kit or MonsterInsights. The Measurement ID is property-specific, so if you manage multiple GA4 properties you'll have different IDs for each. If you don't see Data Streams in your Admin panel, you're likely still looking at a Universal Analytics property. GA4 properties have a different structure and you may need to create a new GA4 property if you haven't migrated yet. Universal Analytics stopped collecting data in July 2023, so every site should be on GA4 by now. At Ottawa SEO, we store each client's Measurement ID in our project documentation because you'll reference it repeatedly when setting up conversion tracking, connecting Search Console, configuring server-side tagging, or troubleshooting tag fires. The ID itself is not sensitive information but treat the associated API keys and service account credentials carefully. One common mistake: confusing the Measurement ID with the Property ID (a plain number visible in Admin settings) or the old UA tracking code. Only the G-XXXXXXXXXX format works for GA4 implementation. If you're managing a 500-domain portfolio like ours, document which Measurement ID belongs to which property because GA4's interface makes it surprisingly easy to lose track when switching between accounts and properties rapidly. Use clear naming conventions in GA4 property names that match your internal asset tracking system.