When oil enters the sea as a result of an oil spill it causes both immediate and long-term changes in the biotic and abiotic environments. This publication is the first uniform Finnish national plan of action in the case of largescale oil pollution incidents. For this report, a working group comprising researchers, experts and authorities has compiled essential current information on the impact of oil on marine ecology as well as guidelines on how to conduct research and sampling. Furthermore, the areas of research responsibilities have been clearly defined. The focus of this report is on the impact of mineral oils. It has been found in connection with earlier oil spills that the impacts of petroleum hydrocarbons on living organisms extend beyond the visibly polluted area. This action plan therefore provides instructions on how to survey the ecological impacts of oil irrespective of the extent of the oil spill, and covering areas beyond the main affected area. In addition to acute-stage research, monitoring of long-term effects and oil levels is also necessary. The Baltic Sea is a particularly vulnerable sea area. The number of oil spills caused by vessels has decreased over the past decades due to tighter requirements regarding vessel condition, strict sanctions on spills and improved overall surveillance and route planning, while the amount of vessel traffic has significantly increased. Over the period from 1995 to 2010, oil transport in the Gulf of Finland increased nearly 10-fold, and this trend is expected to continue as Russia opens new oil terminals and increases the capacity of its existing terminals. Furthermore, it is possible that significant amounts of oil enter the sea from onshore sources, such as industrial plants. This action plan aims to provide guidelines in case of an oil spill for the Finnish authorities responsible for studying the impacts of oil on the marine ecosystems in Finland. The publication is divided into three sections: Section A contains general background information on the impacts of oil spills on the marine environment and approaches required to carry out the impact studies; Section B describes step-by-step instructions on how to initiate the necessary procedures in case of an oil spill; and in Section C, the need for further study on baseline oil levels in marine ecosystems is discussed. Section B may be used as a standalone guide.