This is an in-depth guide on how to diagnose and debug common issues with SAML (Security Assertion Markup Language) assertion failures. The article provides a step-by-step approach, covering topics such as understanding the structure of a SAML assertion, identifying common causes of failure, and using tools like SAML-tracer to capture and decode SAML responses. It also includes troubleshooting scenarios, best practices for configuring IdP and SP settings, and advice on how to handle certificate management and metadata updates. The guide aims to empower developers with the knowledge and skills needed to successfully implement and troubleshoot SAML-based Single Sign-On (SSO) integrations.