This book constitutes the refereed proceedings of the 11th International Conference on Modelling Tools and Techniques for Computer Communication System Performance Evaluation, TOOLS 2000, held in Schaumburg, IL, USA in March 2000. The 21 revised full papers presented were carefully reviewed and selected from a total of 49 submissions. Also included are 15 tool descriptions and one invited paper. The papers are organized in topical sections on queueing network models, optimization in mobile networks, stochastic Petri nets, simulation, formal methods and performance evaluation, and measurement tools and applications.