This book constitutes the refereed proceedings of the 13th International Conference on Modelling Techniques and Tools for Computer Performance Evaluation, TOOLS, 2003, held in Urbana, IL, USA, in September 2003. The 17 revised full papers presented together with a keynote paper were carefully reviewed and selected for inclusion in the book. The papers are organized in topical sections on tools for measuring, benchmarking, and online control; tools for evaluation of stochastic models; queueing models; Markovian arrival processes and phase-type distributions; and supporting model-based design of systems.