This book constitutes the refereed proceedings of the 18th European Conference on Object-Oriented Programming, ECOOP 2004, held in Oslo, Norway in June 2004.The 25 revised full papers presented together with the abstracts of 2 invited talks were carefully reviewed and selected from a total of 132 submissions. The papers are organized in topical sections on encapsulation, program analysis, software engineering, aspects, middleware, types, verification, and systems.