This book constitutes the joint refereed post-conference proceedings of 18 workshops held in conjunction with the 12th European Conference on Object-Oriented Programming, ECOOP'98, held in Brussels, Belgium, in July 1998. Also included are revised demos and posters presented at the conference. The volume presents around 250 revised contributions, including surveys by the respective workshop organizers and abstracts of the individual contributions. The wealth of up-to-date information provided spans the whole spectrum of object technologies, from theoretical and foundational issues to applications in a variety of domains.