Security is essential in software development, and one effective approach is the zero trust model, which assumes "never trust, always verify" for every request to access resources. This model provides a comprehensive framework for protecting sensitive data and resources by verifying explicitly, using least privilege access, and assuming breach. Implementing zero trust principles in API development requires a combination of technical measures, best practices, and a shift in mindset, including authentication and authorization, encryption, API gateways, monitoring and logging, regular security audits, and rotating credentials regularly. Best practices for APIs built on zero trust principles include designing with simplicity, automating security checks, prioritizing clear documentation, and rotating and refreshing credentials. Zero trust also applies to event-driven or async APIs by validating events, controlling access, maintaining audit logs, applying rate limiting, and encrypting sensitive data. The zero trust API maturity model provides a framework for assessing current implementation levels and identifying areas for improvement, with four stages representing different levels of maturity: traditional, initial, advanced, and optimal. By applying zero trust principles to API development, developers can ensure their APIs are secure from the ground up.