Security protocols are distributed programs that aim at securing communications by the means of cryptography. They are for instance used to secure electronic payments, home banking and more recently electronic elections. Given the financial and societal impact in case of failure, and the long history of design flaws in such protocols, formal verification is a necessity. A major difference from other safety critical systems is that the properties of security protocols must hold in the presence of an arbitrary adversary. The aim of these notes is to provide a tutorial to some modern approaches for formally modeling protocols, their goals and automatically verifying them.