The allocation of liability for environmental harm in areas beyond national jurisdiction (ABNJ) is complicated by several factors relating to the nature of the activities undertaken and the nature of environmental harm itself. These include the range of actors involved in activities that may give rise to environmental harm, and features of cumulative environmental damage, arising over a course of time either out of a connected or unconnected set of activities involving multiple actors or from external natural causes. To address these issues, this chapter outlines legal approaches to causation and the challenges that complex causal pathways may present in ABNJ. It then discusses the general approach to allocating responsibility to states and international organizations under international law and national law, and the channeling of liability to operational entities which is the principal approach in sector-specific civil liability regimes. Finally, it explores the rules that structure the allocation of liability in relation to specific ABNJ regimes and activities.