This chapter introduces the Message Authentication Code (MAC), a crucial cryptographic primitive designed to simultaneously ensure data integrity and verify message origin. While standard cryptographic hash functions effectively detect unauthorized data alterations, they lack the capacity to authenticate the sender&s;s identity. To resolve this inherent limitation, MACs integrate a shared secret key into the verification process. The text systematically explores the primary paradigms of message authentication, detailing encryption-based methods, hash-based constructions (often termed keyed hash functions, such as HMAC), and distinct authentication algorithms based on block ciphers (such as CBC-MAC). Furthermore, the chapter provides rigorous formal definitions and security analyses for these constructions. Concluding with an essential examination of true and pseudorandom number generation, this chapter equips readers with a comprehensive understanding of the mechanisms utilized to prevent forgery and authenticate communications within secure digital networks.