Offshore wind farms create a new habitat in the ocean where marine animals can find food and shelter. However, operational wind turbines generate a continuous noise that risks negatively affecting animals by masking important sounds or affecting behaviour. A noise assessment shall be carried out to get an estimate of the spatial and temporal scale of this impact. This study aims to provide an overview of operational noise from offshore wind turbines, which factors affect the noise level and how this noise can affect marine organisms with a focus on porpoises and fish: cod and herring and their eggs and larvae. This has been done through a literature and modelling study where the noise level from todays and future wind turbines has been calculated. Source models were developed for turbines with and without gearboxes and at wind speeds of 7 and 15 m/s respectively. These were used to make noise maps at different locations. An assessment method was developed for the impact on fish and porpoises, which includes impact levels for masking and behavioural effects. However, these levels are uncertain as only a few studies report at which sound levels the animals have been affected.