The mobile app authentication best practices question is the single hardest one to answer well in mobile application security, because the answers that work for web applications fail in subtle ways on mobile devices. The browser does most of the heavy lifting in a web application's authentication flow — cookie handling, redirect orchestration, session storage with reasonable defaults. The mobile application has to build the equivalent capability itself, against a platform where the application binary is in the attacker's possession, where biometric prompts are often treated as authentication when they should be treated as presence assertions, where token storage decisions made at design time bind the application's security posture for its entire deployment, and where the OAuth flow patterns appropriate for confidential web clients are categorically wrong for public mobile clients. This guide walks the best practices for secure authentication in mobile apps from a developer's perspective, anchored to the OWASP mobile application security verification standard (MASVS) V4 chapter that defines the verification requirements at L1, L2, and L2+R levels. The broader mobile security context — platform security primitives, the OWASP Mobile Top 10 2024, the complete attack surface — is covered in the mobile app security pillar; this deep dive focuses on the authentication subset specifically.