Inter-municipal co-operation is a widespread phenomenon throughout Europe. But in spite of its wide spread, inter-municipal co-operation has not been subject to systematic comparative research. This article presents some of the findings of a joint research project concerning inter-municipal co-operation in a series of European countries. A comparative analysis shows that inter-municipal co-operation comes in all shapes and sizes. Nevertheless it is possible to distinguish four basic types of co-operation: quasi-regional governments, planning forums, service delivery organizations and service delivery agreements. Research shows that the national institutional context to a large extent explains the presence or absence of the different types in a country.