Urban flood risk is a big and growing concern, particularly in relation to climate change. The complexity of this problem poses distinctive challenges to a broad range of actors involved in various ways in flood risk governance, as well as to the social scientists endeavoring to grasp them. These challenges are related both to the complexity of risk and risk perception as such, which attract a lot of attention, and in relation to the complexity of flood risk governance. This paper focuses on the latter. Floods tend not to be bounded by geopolitical or administrative borders, and involve various sectors in society. Flood risk must thus be jointly governed by a complex web of actors, who are not independent of each other but dependent on various resources and affected by the decisions and actions others are making. The patterns of social relations among these actors are therefore fundamental for society's capacity to reduce risk.