s2n-tls is a C99 implementation of the TLS/SSL protocols designed by AWS for simplicity, small size, and security. It supports TLS 1.2 and 1.3, including features like 0-RTT early data, session tickets, post-quantum KEMs (ML-KEM, ML-DSA), and QUIC transport integration. The library provides a public C API (api/s2n.h) and multiple libcrypto backends (OpenSSL, AWS-LC, BoringSSL, LibreSSL). Extensive formal verification (CBMC, SAW, SideTrail) and fuzz testing ensure memory safety and constant-time execution. The architecture is highly modular with a stuffer buffer abstraction, blob/array utilities, a table-driven TLS extension framework, and a robust error-handling macro system. Rust bindings are first-class, offering safe wrappers (s2n-tls), async Tokio integration (s2n-tls-tokio), an HTTP client connector for Hyper (s2n-tls-hyper), and a metrics subscriber for handshake analysis. The library is used by AWS services like S3 and KMS, and includes interoperability tests against OpenSSL, GnuTLS, Java, and rustls. It is actively maintained with a sophisticated CI pipeline covering unit, integration, fuzz, formal proof, and performance regression tests across multiple platforms and libcrypto versions.