This volume is based on papers presented at the5thWorkshoponM- brane Computing, WMC5, which took place in Milan, Italy, in the period June 14¿16, 2004, as a satellite event of DNA10 (10th International Workshop on DNA-Based Computing). The ?rst three workshops were organized in Curtea de Arge¿ s, Romania ¿ they took place in August 2000 (with the proceedings p- lished in Lecture Notes in Computer Science, volume 2235), in August 2001 (with a selection of papers published as a special issue ofFundamenta Inform- icae, volume 49, numbers 1¿3, 2002), and in August 2002 (with the proceedings publishedinLectureNotesinComputerScience,volume2597).Thefourthwo- shop took place in Tarragona, Spain, in July 2003 (the proceedings appeared as volume 2933 of Lecture Notes in Computer Science). Like the previous two meetings, also WMC5 was an o?cial workshop of the Molecular Computing Network (MolCoNet) funded by the EU Commission in theFifthFrameworkprogramInformationSocietyTechnologies(projectnumber IST-2001-32008). The preproceedings of WMC5 were published as a MolCoNet report, and they were available during the workshop. This volume contains only a selection of the papers from the preproceedings.

Invited Lectures.- ?P Systems and Typed ?-Calculus.- P Automata.- Asynchronous P Systems and P Systems Working in the Sequential Mode.- Evolution and Oscillation in P Systems: Applications to Biological Phenomena.- An Approach to Computational Complexity in Membrane Computing.- LMNtal: A Language Model with Links and Membranes.- Regular Presentations.- Executable Specifications of P Systems.- On the Efficiency of P Systems with Active Membranes and Two Polarizations.- Communicative P Systems with Minimal Cooperation.- Ultimately Confluent Rewriting Systems. Parallel Multiset-Rewriting with Permitting or Forbidding Contexts.- Unstable P Systems: Applications to Linguistics.- A P System Description of the Sodium-Potassium Pump.- Inhibiting/De-inhibiting Rules in P Systems.- Time-Independent P Systems.- On Two-Dimensional Mesh Networks and Their Simulation with P Systems.- Exploring Computation Trees Associated with P Systems.- Approximating Non-discrete P Systems.- Reducing the Size of Extended Gemmating P Systems.- P Systems Generating Trees.- On Descriptive Complexity of P Systems.- P Systems with Symport/Antiport: The Traces of RBCs.- Conservative Computations in Energy-Based P Systems.- General Multi-fuzzy Sets and Fuzzy Membrane Systems.- Trading Polarization for Bi-stable Catalysts in P Systems with Active Membranes.- Modelling Dynamic Organization of Biology-Inspired Multi-agent Systems with Communicating X-Machines and Population P Systems.- On the Size of P Systems with Minimal Symport/Antiport.