Next.js has emerged as a leading framework for JavaScript and React development due to its exceptional developer experience, performance optimization capabilities, and versatile rendering options. The release of Next.js 13 introduced the "App Router," shifting to a React Server Components-based architecture, which enhances performance but alters the authentication process. This change necessitates an updated understanding of authentication in Next.js, guiding developers on how to implement secure user authentication in their applications. The App Router supports various rendering strategies like Client-Side Rendering (CSR), Static Site Generation (SSG), Incremental Static Regeneration (ISR), and Server-Side Rendering (SSR), each serving distinct use cases from dynamic dashboards to static marketing pages. Developers can choose between the traditional Pages Router and the new App Router, with Clerk providing authentication support for both, allowing a seamless transition while ensuring optimal data protection. The article provides detailed instructions on setting up authentication in both routing paradigms, helping developers maintain secure and efficient applications.