App Router (Next.js)
Next.js routing system launched in 13 (Oct 2022). Replaces Pages Router. File-system-based: /app/posts/[slug]/page.tsx = /posts/:slug. Native React Server Components support. Layouts, loading, error boundaries per route.
Major mental shift from Pages Router: components are server by default (no JS shipped unless 'use client'). Streaming rendering. Parallel + intercepting routes. Server Actions integration. Adoption friction: large migration effort for existing Pages projects, mental-model complexity. Next.js 15 (2024): App Router considered stable, recommended default. Vercel developer survey 2024: ~60% of new projects start with App Router. Pages Router still supported but a deprecated path eventually. Documentation 2024+: App Router examples by default.