Incremental Static Regeneration (ISR) is a Next.js feature that lets pre-rendered pages refresh on a schedule (or on-demand) without a full site rebuild. The page is served from the cache; when stale, the next request triggers a background regeneration; subsequent requests get the fresh version.
For content sites with hundreds of pages and frequent edits, ISR is the architectural sweet spot: edge-cached performance + editorial freshness within minutes. The on-demand variant (Next.js revalidateTag + a CMS webhook) drops staleness to seconds when an editor hits "publish".
The discipline that makes ISR safe at scale: cache tags scoped per content type, webhook signing on the revalidation endpoint, and a documented fallback when the upstream CMS is unreachable (stale-while-revalidate keeps the old version live).