As part of the Department of Trade and Industry's (now Department of Energy and Climate Change) Strategic Environmental Assessment (SEA2), this report reviews pockmarks in the North Sea. Pockmarks frequently contain unusual densities of fish, typically large gadoids such as cod, torsk and ling. High densities of fish otoliths have been found in one North Sea pockmark. The existence of cemented sediment in the base of pockmarks can provide a refuge for fish and a hard substrate for colonisation by epifauna such as anthozoa. Deep active pockmarks, such as the three studied in block 15/25, are known to contain species dependent on high sulphide concentrations, originating from seepage or enhanced sedimentation. Some of these species are not found elsewhere in the open North Sea. The 20 m deep "Scanner" pockmark in block 15/25 is the only pockmark in the UK sector of the North Sea whose biology has been studied in detail. It is the only known habitat of the gutless nematode Astononema southwardorum.