What are the key differences between Next.js 13, 14, and 15?

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Each of these releases introduced a major architectural shift. A quick tour of what changed and why it matters. Next.js 13 (October 2022) — introduced the App Router in beta. The biggest release in the framework's history: app/ directory — file-system routing that uses React Server Components by default. Nested layouts and loading states — layout.tsx , loading.tsx , error.tsx , not-found.tsx . Streaming and Suspense — pages can stream from the server, showing UI incrementally. Server Components, Client Components — clear split between server-only and client-executable code. Turbopack (alpha) — Rust-based dev bundler, much faster than Webpack. next/image and next/font — redesigned image component and first-class web-font support. Next.js 14 (October 2023) — stabilized 13's new features, no breaking changes: Server Actions (stable) — 'use server' functions callable from forms and client code. Partial Prerendering (preview) — static shells with streaming dynamic content in the same page. Turbopack for dev — significantly faster local startup and HMR for most projects. Metadata improvements — dynamic OG image generation via ImageResponse . Bundle size and memory usage reductions across the App Router runtime. Next.js 15 (October 2024) — shifted defaults around caching and async APIs: Async request APIs — cookies() , headers() , params , and searchParams are now async. Existing code needs await . Caching defaults inverted — fetches and Route Handlers are now uncached by default. Opt back into caching explicitly. React 19 support — Actions, use() , and new form features all work in App Router. Turbopack stable for dev — default bundler for next dev . Static Route Indicator and Dev Instrumentation — clearer diagnostics in dev. next/after — schedule work after the response has been sent. Partial Prerendering (still experimental) — production-ready for many apps. Practical migration notes: 13 to 14 — generally drop-in. Adopt Server Actions if you're doing forms. 14 to 15 — requires code changes. The codemod command automates most of the async API migrations: npx @next/codemod@latest next-async-request-api . Audit every fetch call to decide whether it should be cached. Anything older than 13 — the Pages Router still works and is still maintained. Moving to the App Router is encouraged but not required for existing projects. This is a frequently asked question. Know the headline feature of each version: v13 = App Router (beta), v14 = stable App Router + Server Actions, v15 = default caching changes + async APIs. The v15 caching change is especially important to mention.