The marine renewable energy industry is expanding globally in response to increased energy demands and the desire to curtail greenhouse gas emissions. Within the UK, Wales has the potential for the development of diverse marine renewable technologies, with a strong tidal range resource, areas of high tidal current energy, and a spatially limited wave energy resource. Targets have been set by the Welsh Government to increase the contribution of marine renewable energy to Wales' electricity generation, and the recent introduction of demonstration zones for tidal and wave energy aims to facilitate developers in device deployment. However, uncertainties remain about the potential impacts of devices, particularly for array scale deployments, planned at several sites, and for the extensive structures required to capture the tidal range resource. Here we review present knowledge of potential impacts, including physical, ecological and societal dimensions, and outline research priorities to provide a scientific basis on which to base decisions influencing the trajectory of Welsh marine renewable energy development.