The present volume contains a selection of papers presented at the Seventh Workshop on Membrane Computing, WMC7, which took place in Leiden, The Netherlands, during July 17-21, 2006. The ?rst three workshops on membrane computing were organized in Curtea de Arge s, Romania – they took place in August 2000 (with the proceedings published in Lecture Notes in Computer Science, volume2235), in August 2001(with a selection of papers published asa special issue of Fundamenta Informaticae, volume 49, numbers 1-3, 2002), and in August 2002 (with the proceedings published in Lecture Notes in Computer Science, volume 2597). The next three workshops were organized in Tarragona, Spain, in July 2003, in Milan, Italy, in June 2004, and in Vienna, Austria, in July 2005, with the proceedings published as volumes 2933, 3365, and 3850, respectively, of Lecture Notes in Computer Science. The 2006edition of WMC wasorganized(and supported) by Lorentz Center, Leiden, under the auspices of the European Molecular Computing Consortium (EMCC). Special attention was paid to the interaction of membrane computing with biology, focusing both on the biological roots of membrane computing and on applications of membrane computing in biology and medicine. Furthermore, the meeting was planned also as an event promoting the interaction and co- eration between the participants (e. g. , the workshop was one day longer than usually, with afternoons devoted mainly to joint work). The pre-proceedings of WMC7 were published by the Institute of Advanced Computer Science (LIACS) of Leiden University, and they were availableduring theworkshop.