Intergovernmental co-operation among Swiss cantons is considered to be much more intensive than in many other federal states. This article investigates different explanations for when and why subnational entities co-operate with each other, and tests them on the Swiss case. In the Swiss case, co-operation seems to be closely connected to the small-scale structure of subnational units. Swiss cantons are fairly small, often smaller than the relevant areas for the provision of public services. This means that cantons need to co-operate with their neighbours and proximate cantons. Further, in the absence of an administrative level that unifies areas with a common language, concordats are a means to co-ordinate policies among German-speaking and French-speaking cantons. The different partisan colours of the cantonal governments only marginally hinder them from co-operating with each other.