This text constitutes the refereed proceedings of the 12th International Conference on Modelling Techniques and Tools for Computer Performance Evaluation, TOOLS 2002, held in London, UK in April 2002. The 18 revised full papers and six tool papers presented together with an invited contribution were carefully reviewed and selected from 57 submissions. Among the topics addressed are generic techniques like stochastic process algebras and the analysis of Petri nets and Markov chains, as well as the development and employment of tools in areas such as the Internet, software performance...