We are pleased to present the proceedings of Performance TOOLS 2003, the 13th International Conference on Modelling Techniques and Tools for Computer Performance Evaluation. The series of TOOLS conferences has provided a forum for our community of performance engineers with all their diverse interests. TOOLS 2003, held in Urbana, Illinois during September 2-5, 2003, was the most recent meeting of the series, which in the past has been held in the following cities: 1984 Paris 1992 Edinburgh 2000 Chicago 1985 Sophia-Antipolis 1994 Vienna 2002 London 1987 Paris 1995 Heidelberg 2003 Urbana 1988...