This book constitutes the refereed proceedings of the 12th European Conference on Object-Oriented Programming, ECOOP'98, held in Brussels, Belgium, in July 1998. The book presents 24 revised full technical papers selected for inclusion from a total of 124 submissions; also presented are two invited papers. The papers are organized in topical sections on modelling ideas and experiences; design patterns and frameworks; language problems and solutions; distributed memory systems; reuse, adaption and hardware support; reflection; extensible objects and types; and mixins, inheritance and type analysis complexity.