This book constitutes the thoroughly refereed extended postproceedings of the 8th International Workshop on Membrane Computing, WMC 2007, held in Thessaloniki, Greece, in June 2007 under the auspices of the European Molecular Computing Consortium (EMCC). The 22 revised full papers presented together with 5 invited papers went through two rounds of reviewing and improvement. The papers in this volume cover all the main directions of research in membrane computing, ranging from theoretical topics in mathematics and computer science to application issues. A special attention was paid to the interaction of membrane computing with biology and computer science, focusing both on the biological roots of membrane computing, on applications of membrane computing in biology and medicine, and on possible electronically based implementations. This volume contains a selection of papers presented at the Eighth Workshop on Membrane Computing, WMC8, which took place in Thessaloniki, Greece, during June 25¿28, 2008. 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,volume 2235), in August 2001 (with a selection of papers published as a 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 four workshops were organized in Tarragona, Spain, in July 2003, in Milan, Italy, in June 2004, in Vienna, Austria, in July 2005, and in Leiden, The Netherlands, in July 2006, with the proceedings published as volumes 2933, 3365, 3850, and 4361, respectively, of Lecture Notes in Computer Science. The 2007 edition of WMC was organized by the South-East European - search Centre in Thessaloniki, under the auspices of the European Molecular Computing Consortium (EMCC). Special attention was paid to the interaction of membrane computing with biology and computer science, focusing on the - ological roots of membrane computing, on applications of membrane computing in biology and medicine, and on possible electronically based implementations. The pre-proceedings of WMC8 were published by the South-East European Research Centre, Thessaloniki, and they were available during the workshop. Each paper was refereed by two members of the Program Committee. Invited Lectures Psim: A Computational Platform for Metabolic P Systems 1 Luca Bianco and Alberto Castellini Modeling the Dynamics of HIV Infection with Conformon-P Systems and Cellular Automata 21 Pierluigi Frisco and David Wolfe Corne (UREM) P Systems with a Quantum-Like Behavior: Background, Definition, and Computational Power 32 Alberto Leporati The Calculus of Looping Sequences for Modeling Biological Membranes 54 Roberto Barbuti, Andrea Maggiolo Schettini, Paolo Milazzo, and Angelo Troina Membrane Computing in Connex Environment 77 Mihaela Malita and Gheorghe Stefan Regular Papers Skin Output in P Systems with Minimal Symport/Antiport and Two Membranes 97 Artiom Alhazov and Yurii Rogozhin On the Reachability Problem in P Systems with Mobile Membranes 113 Bogdan Aman and Gabriel Ciobanu Modeling Symport/Antiport P Systems with a Class of Hierarchical Petri Nets 124 Luca Bernardinello, Nicola Bonzanni, Marco Mascheroni, and Lucia Pomello A Hybrid Approach to Modeling Biological Systems 138 Francesco Bernardini, Marian. Gheorghe, Francisco Jos omero-Campero, and Neil Walkinshaw Causality in Membrane Systems 160 Nadia Busi Simulating the Bitonic Sort Using P Systems 172 Rodica Ceterchi, Mario J. P z-Jim z, and Alexandru Ioan Tomescu On the Number of Agents in P Colonies 193 Ludek Cienciala, Lucie Ciencialov and Alica Kelemenov TD> Events, Causality, and Concurrency in Membrane Systems 209 Gabriel Ciobanu and Dorel Lucanu P Systems with String Objects and with Communication by Request 228 Erzs t Csuhaj-Varj and Gy rgy Vaszil On the Dynamics of PB Systems with Volatile Membranes 240 Giorgio Delzanno and Laurent Van Begin A Logarithmic Bound for Solving Subset Sum with P Systems 257 Daniel Diaz-Perna, Miguel A. Guti ez-Naranjo, Mario J. P z-Jim z, and Agustin, Riscos-N ez A Formal Framework for Static (Tissue) P Systems 271 Rudolf Freund and Sergey Verlan Conformon-P Systems with Negative Values 285 Pierluigi Frisco Optimizing Evolution Rules Application and Communication Times in Membrane Systems Implementation 298 Jorge A. Tejedor, Abraham Guti ez, Luis Fern ez, Fernando Arroyo, Gin Bravo, and Sandra G mez Hill Kinetics Meets P Systems: A Case Study on Gene Regulatory Networks as Computing Agents in silico and in vivo 320 Thomas Hinze, Sikander Hayat, Thorsten Lenser, Naoki Matsumaru, and Peter Dittrich Solving Numerical NP-Complete Problems with Spiking Neural P Systems 336 Alberto Leporati, Claudio Zandron, Claudio Ferretti, and Giancarlo Mauri Towards a Complete Covering of SBML Functionalities 353 Tommaso Mazza Active Membrane Systems Without Charges and Using Only Symmetric Elementary Division Characterise P 367 Niall Murphy and Damien Woods Balancing Performance, Flexibility, and Scalability in a Parallel Computing Platform for Membrane Computing Applications 385 Van Nguyen, David Kearney, and Gianpaolo Gioiosa On Flip-Flop Membrane Systems with Proteins 414 Andrei Paun and Alfonso Rodriquez-Pat n Characterizing Membrane Structures Through Multiset Tree Auomata 428 Jos . Sempere and Dami L pez OPERAScc: An Instance of a Formal Framework for MAS Modeling Based on Population P Systems 438 Joanna Starriatopoulou, Petros Kefalas, and Marian Gheorghe Author Index 453