This book constitutes the refereed proceedings of the 13th International Workshop on Power and Timing Modeling, Optimization and Simulation, PATMOS 2003, held in Torino, Italy in September 2003. The 43 revised full papers and 18 revised poster papers presented together with three keynote contributions were carefully reviewed and selected from 85 submissions. The papers are organized in topical sections on gate-level modeling and characterization, interconnect modeling and optimization, asynchronous techniques, RTL power modeling and memory optimization, high-level modeling, power-efficient technologies and designs, communication modeling and design, and low-power issues in processors and multimedia.