This book constitutes the refereed proceedings of the 21st European Conference on Object-Oriented Programming, ECOOP 2007, held in Berlin, Germany in July/August 2007.

The 25 revised full papers, presented together with 3 invited talks were carefully reviewed and selected from a total of 135 final submissions. The papers are organized in topical sections on types, runtime implementation, empirical studies, programs and predicates, language design, inheritance and derivation, aspects, as well as language about language.