The OWASP Top 10 (2021) Comprehensive Guide provides a detailed overview of the ten most critical web application security risks, identifying prevalent and impactful vulnerabilities. It outlines various categories such as Broken Access Control, Cryptographic Failures, and Injection, explaining their technical details, common attack scenarios, and real-world case studies. The guide offers practical prevention techniques, including code examples and recommended tools, to help developers and organisations mitigate these risks effectively. Emphasising the importance of secure design and continuous monitoring, it serves as a crucial resource for building and maintaining robustly secure applications.