This book constitutes the thoroughly refereed post-workshop proceedings of the 19th International Workshop on Security Prools, held in Cambridge, UK, in March 2011. Following the tradition of this workshop series, each paper was revised by the authors to incorporate ideas from the workshop, and is followed in these proceedings by an edited transcription of the presentation and ensuing discussion. The volume contains 17 papers with their transcriptions as well as an introduction, i.e. 35 contributions in total. The theme of the workshop was "Alice doesn't live here anymore".