Plans for Marine Renewable Energy Installations (MREI) are developing worldwide, yet many questions still remain about the impacts such developments may have on marine ecosystems and on coastal and oceanographic processes. This uncertainty, combined with a lengthy and complex Environmental Impact Assessment (EIA) phase prior to consent, has slowed the growth of the marine renewables sector. Information on completed and ongoing EIAs at MREI sites across Europe was summarised and compared amongst sites and with completed, comprehensive EIAs for Horns Rev offshore wind farm and the SeaGen tidal turbine site at Strangford Lough. This allowed for the identification of commonalities and differences in monitoring activities, and of data gaps in the wave energy EIA process. Studies on the socio-economic impacts of MREIs were lacking, as were monitoring of fish, fish habitats, electromagnetic fields and their impacts on marine wildlife. Even amongst sites monitoring similar topics, methodologies varied greatly. Science cannot inform the management of marine renewables whilst there are inconsistencies in baseline and impact monitoring, as this study has documented. A streamlined EIA process and collaborations between researchers and developers are required to move the industry forward.