This book constitutes the refereed proceedings of the first International Conference on Principles of Security and Trust, POST 2012, held in Tallinn, Estonia, in March/April 2012, as part of ETAPS 2012, the European Joint Conferences on Theory and Practice of Software. The 20 papers, presented together with the abstract of an invited talk and a joint-ETAPS paper, were selected from a total of 67 submissions. Topics covered by the papers include: foundations of security, authentication, confidentiality, privacy and anonymity, authorization and trust, network security, protocols for security, language-based security, and quantitative security properties.