In data transmission, security is an indispensable cornerstone. The Transport Layer Security (TLS) protocol ensures the confidentiality and integrity of application-layer communications across the entire network, forming the core of modern internet security. This chapter focuses on TLS, providing an in-depth explanation of its core security capabilities: data confidentiality, data integrity, and authentication. It details the TLS handshake, comparing traditional handshakes with the efficient 1-RTT and 0-RTT mechanisms in TLS 1.3. The chapter also addresses common security threats like man-in-the-middle attacks and replay attacks. Finally, a hands-on experiment analyzing TLS handshake packet captures helps students grasp TLS’s critical role in protecting transmitted data.