This book constitutes the refereed proceedings of the 20th European Conference on Object-Oriented Programming, ECOOP 2006, held in Nantes, France in July 2006. 20 revised full papers, together with 3 keynote papers were carefully reviewed and selected. The papers are organized in topical sections on program query and persistence, ownership and concurrency, languages, type theory, types for object-oriented languages, tools, and modularity. 5 more papers celebrate the 20th anniversary of ECOOP.