GCLID (Google Click Identifier) is a unique code Google adds to a URL when someone clicks your ad. Together with similar identifiers from other platforms (e.g. fbclid for Meta), it's the glue that ties an ad click to a later conversion.
How it works
When a visitor arrives from an ad, a parameter like ?gclid=... appears in the address. Measurement captures and stores this identifier. At conversion time it attaches it, so the platform knows which click (and therefore campaign, ad group, keyword) the conversion belongs to.
Why they matter
- Accurate attribution. Without a click ID, the platform would have to match only via cookies, which ad-blockers and ITP weaken.
- Offline conversions. This is key for longer cycles (B2B, SaaS, expensive goods): a stored GCLID lets you send the conversion back to Google Ads even when it happens off-site — for example when a lead in the CRM matures into a deal.
How not to lose them
- Capture the click ID on arrival and store it (e.g. in a first-party cookie or with the customer record).
- Watch redirects. Misconfigured redirects can drop the parameter from the URL — and then the click ID is gone.
- Use longer-lived identification. For long journeys, server-side helps the link between click and conversion survive.
How to use click IDs for long sales cycles is covered in lead and B2B measurement with server-side. For the connection to conversion matching, see Enhanced Conversions for Google Ads.