This book constitutes the refereed proceedings of the 13th European Conference on Object-Oriented Programming, ECOOP'99, held in Lisbon, Portugal in June 1999. The 20 revised full papers presented were carefully reviewed and selected from a total of 183 submissions on the basis of three to five reviews. Also included are four invited papers. The book is structured in topical sections on mixins, debugging and garbage collection, type checking, virtual and multi-methods, adaptive programming, classification and inheritance, and distributed objects.