This text on the costs of prevention and reparation of environmental harm analyzes the tension between economic and legal principles at state level. It considers traditional thinking on state liability for transboundary harm, and the theories which have challenged it since the proliferation of hazardous activities in the 1960s. The author favours a return to traditional thinking, but gives due weight to theories that challenged it with the aim of safeguarding the compensation of victims of transboundary harm.