ECOOP '91 is the fifth annual European Conference on Object-Oriented Programming. From their beginning, the ECOOP conferences have been very successful as a forum of high scientific quality where the newest devel- opments connected to object-oriented programming and related areas could be presented and discussed. Over the last few years object-oriented technology has gained widespread use and considerable popularity. In parallel with this, the field has matured scientifically, but there is still a lot of room for new ideas and for hot debates over fundamental issues, as these proceedings show. The 22 papers in this volume were selected by the programme committee from 129 submissions. Important issues discussed in the contributions are language design, specification, databases, concurrency types and software development.