The purpose of this survey of St. Kilda SAC sea caves, undertaken between the 15th and 17th August 2015, was to initiate site condition monitoring (SCM). The objectives of SCM were to establish a baseline biological data set that would facilitate the assessment of the condition of habitats in the future and to allow a judgement to be formed on the current condition of these habitats in the light of existing SAC management measures. At 4 selected sea caves, detailed biological and physical surveys were conducted; the topography was mapped and the biotopes surveyed by MNCR phase 2 methodology at relocatable points along the cave system. When wave surge prevented access to the caves, 3 rocky reef sites were targeted with the aim of surveying the biota along a relocatable transect running contiguously from the supralittoral into the sublittoral. Two other sea cave surveys were conducted as part of the same research cruise at North Rona SAC and Loch Eriboll.