This book constitutes the thoroughly refereed post-conference proceedings of the 13th International Conference on Membrane Computing, CMC 2012, held in Budapest, Hungary, in August 2012. The 21 revised selected papers presented together with 6 invited lectures were carefully reviewed and selected from 25 papers presented at the conference. The book also deals with membrane systems, also called P systems, which are distributed and parallel algebraic models processing multisets of objects in a localized manner (evolution rules and evolving objects are encapsulated into compartments delimited by membranes), with an essential role played by the communication among compartments and with the environment.