Principles of Security and Trust is a broad forum related to the theoretical and foundational aspects of security and trust. Papers of many kinds are welcome: new theoretical results, practical applications of existing foundational ideas, and innovative theoretical approaches stimulated by pressing practical problems. POST combines and replaces a number of successful and longstanding workshops in this area: Automated Reasoning and Security Protocol Analysis (ARSPA), Formal Aspects of Security and Trust (FAST), Security in Concurrency (SecCo), and the Workshop on Issues in the Theory of Security (WITS). A subset of these events met jointly as an event affiliated with ETAPS 2011 under the name Theory of Security and Applications (TOSCA). We seek submissions proposing theories to clarify security and trust within computer science; submissions establishing new results in existing theories; and also submissions raising fundamental concerns about existing theories. We welcome new techniques and tools to automate reasoning within such theories, or to solve security and trust problems. Case studies that reflect the strengths and limitations of foundational approaches are also welcome, as are more exploratory presentations on open questions.