Google Analytics 4 replaced the old UA tracking ID format with what's called a Measurement ID. This identifier starts with 'G-' and looks like G-12345ABCDE. You find it in your GA4 property settings under Admin > Data Streams > select your web stream, and it's labeled as 'Measurement ID' right at the top. This is different from Universal Analytics, which used tracking IDs starting with 'UA-'. The change reflects how GA4 fundamentally restructured data collection around events rather than sessions and pageviews. The Measurement ID connects your website to a specific data stream in your GA4 property, and a single property can have multiple streams if you're tracking web and app together. When we migrate clients at Ottawa SEO Inc., the most common mistake is leaving the old UA code alongside GA4 without proper configuration. You can run both during a transition period, but each needs its own tag. If you're using Google Tag Manager, you add the Measurement ID to a GA4 Configuration tag. If you're hardcoding it, the gtag.js snippet includes your Measurement ID in the config line: gtag('config', 'G-XXXXXXXXXX'). One thing that trips people up: GA4 properties also have a Property ID (a plain number you see in the UI), but that's not what goes in your tracking code. You need the Measurement ID from the data stream specifically. If you have multiple domains or subdomains, you might create separate data streams with different Measurement IDs, or use one ID with cross-domain tracking configured through GTM. The practical difference for tracking: GA4's Measurement ID only handles web data. If you're tracking iOS or Android apps, those use separate Stream IDs. For most website owners running a single site, you'll have one GA4 property with one web data stream and therefore one Measurement ID to implement. Check your current setup by viewing your page source and searching for 'G-' to see if it's already installed.