Security protocols are the small distributed programs which are omnipresent in our daily lives in areas such as online banking and commerce and mobile phones. Their purpose is to keep our transactions and personal data secure. Because these protocols are generally implemented on potentially insecure networks like the internet, they are notoriously difficult to devise. The field of symbolic analysis of security protocols has seen significant advances during the last few years. There is now a better understanding of decidability and complexity questions and successful automated tools for the provision of security and prevention of attack have been applied to numerous protocols, including industrial protocols. Models have been extended with algebraic properties to weaken the perfect cryptography assumption and even computational soundness results towards cryptographic models have been achieved. What was still missing, however, was a book which summarized the state-of-the-art of these advances. Whilst this book does not pretend to give a complete overview of the field - something which would be impossible in a single volume - it does, nevertheless, cover a representative sample of the ongoing work in this field, which is still very active. The book contains an introduction and ten tutorial-like chapters on selected topics, each written by a leading expert, and will be of interest to all those involved in the formal analysis of security protocols.