ECOOP is the premier forum in Europe for bringing together practitioners, - searchers, and students to share their ideas and experiences in a broad range of disciplines woven with the common thread of object technology. It is a collage of events, including outstanding invited speakers, carefully refereed technical - pers, practitioner reports reffecting real-world experience, panels, topic-focused workshops, demonstrations, and an interactive posters session. The 18th ECOOP 2004 conference held during June 14–18, 2004 in Oslo, Norway represented another year of continued success in object-oriented programming, both as a topic of academic study and as a vehicle for industrial software development. Object-oriented technology has come of age; it is now the commonly established method for most software projects. However, an - panding field of applications and new technological challenges provide a strong demand for research in foundations, design and programming methods, as well as implementation techniques. There is also an increasing interest in the in- gration of object-orientation with other software development techniques. We anticipate therefore that object-oriented programming will be a fruitful subject of research for many years to come. This year, the program committee received 132 submissions, of which 25 were accepted for publication after a thorough reviewing process. Every paper received at least 4 reviews. Papers were evaluated based on relevance, significance, clarity, originality, and correctness. The topics covered include: programming concepts, program analysis, software engineering, aspects and components, middleware, verification, systems and implementation techniques. These were complemented by two invited talks, from Matthias Felleisen and Tom Henzinger. Their titles andabstracts are also included in these proceedings.