This book constitutes the refereed proceedings of the 16th International Conference on Theorem Proving in Higher Order Logics, TPHOLs 2003, held in Rome, Italy in September 2003. The 24 revised full papers presented together with an invited paper were carefully reviewed and selected from 50 submissions. The papers are organized in topical sections on hardware and Assembler languages, proof automation, fool combination, logic extensions, theorem prover technology, mathematical theories, and security. Among the theorem proving systems discussed are HOL, Coq, MetaPRL, and Isabelle/Isar.