Scaling authentication in Next.js applications involves addressing challenges such as session management, database connection limits, edge computing constraints, and performance optimization as user bases grow. Developers face issues like database connection exhaustion, session validation latency, edge runtime incompatibilities, multi-tenancy, and horizontal scaling coordination. Solutions include utilizing connection pooling, stateless JWT authentication, multi-layer caching, and edge-compatible libraries. Managed providers like Clerk, Auth0, Supabase, and Firebase offer scalable authentication solutions with features such as automatic failover, global distribution, and multi-tenancy support, reducing the complexity and resource demands of custom implementations. The decision to use managed providers versus building custom systems depends on specific application needs, team expertise, and long-term maintenance considerations, with managed solutions generally providing faster time-to-market, enterprise features, and robust scalability. As applications scale, maintaining performance, security, and developer productivity becomes essential, and strategic decisions around authentication architecture play a crucial role in supporting growth.