The Vilaine River watershed in Brittany (north-western France) was seriously affected by extreme hydrological events in the latter part of the 20th century, showing the extent to which some urban areas are vulnerable. Such events represent a challenge to land-use planning, particularly in the periphery of fast-growing cities. These outlying areas, analyzed at the local level, are essentially vulnerable right from the onset of development. Considered from the perspective of regulation and climatic constraints, two periods are distinguished. The first is characterized by a relatively non-existent awareness of risk, the other by the tightening of regulations requiring that risk be taken into account.