This volume is based on papers presented at the Workshop on Membrane Computing, WMC 2003, which took place in Tarragona, Spain, in the - riod July 17-July 22, 2003. This was the Fourth Annual Membrane Computing Workshop, and the ?rst one held outside Romania. The ?rst three meetings were organized in Curtea de Arge, s, Romania - they took place in August 2000 (with the proceedings published in Lecture Notes in Computer Science, Vol. 2235), in August 2001 (with a selection of papers published as a special issue of F- damenta Informaticae, Vol. 49, Nos. 1-3, 2002), and in August 2002 (with the proceedings published in Lecture Notes in Computer Science, Vol. 2597). The 2003 workshop was the second workshop of the Molecular Computing Network (MolCoNet) funded by the EU Commission in the Fifth Framework Program Information Society Technologies (project number IST-2001-32008). The preproceedings of WMC 2003 were published as Technical Report 28/03 of theResearchGrouponMathematicalLinguisticsfromRoviraiVirgiliUniversity, Tarragona, and they were available during the workshop.