Authentication protocols are very simple distributed algorithms whose purpose is to enable two entities to achieve mutual and reliable agreement on some piece of information, typically the identity of the other party, its presence, the origin of a message, its intended destination. Achieving the intended agreement guarantees is subtle because they typically are the result of the encryption/decryption of messages composed of different parts, with each part providing a “piece” of the authentication guarantee. This tutorial paper presents the basics of authentication protocols and illustrates a specific technique for statically analysing protocol specifications. The technique allows us to validate protocols in the presence of both malicious outsiders and compromised insiders, with no limitation on the number of parallel sessions. This paper covers the course “Static Analysis of Authentication” given by the author at the FOSAD’04 school. The static analysis technique described here is a joint work with Michele Bugliesi and Matteo Maffei (Università di Venezia)