Protocol Composition Logic (PCL) is a logic for proving authentication and secrecy properties of network protocols. This chapter presents the central concepts of PCL, including a protocol programming language, the semantics of protocol execution in the presence of a network attacker, the syntax and semantics of PCL assertions, and axioms and proof rules for proving authentication properties. The presentation draws on a logical framework enhanced with subtyping, setting the stage for mechanizing PCL proofs. and gives a new presentation of PCL semantics involving honest and unconstrained principals. Other papers on PCL provide additional axioms, proof rules, and case studies of standardized protocols in common use.