Understanding the complexity of environmental impacts of tidal and wave energy converters (TECs, WECs) still presents a major challenge to the expansion of the marine renewable energy (MRE) industry, particularly for new developments. Using the stressor-receptor framework, we broadly introduce the main environmental effects and potential impacts that are considered for TEC and WEC developments. We first provide an overview of the legislation that governs the need to consider the environmental impacts, and the diverse approaches taken to assess them. We then outline potential effects of relevance to the abiotic and biotic environment in the vicinity of TECs and WECs. These include receptor responses to changes in hydrodynamics and sediments, habitat modification, animal collision risk with dynamic parts of devices, and energy emissions including receptor responses to noise and electromagnetic fields associated with installations. We provide an overview of how changes may directly and indirectly influence components of the ecosystem (e.g., habitats, species, processes). In doing so, we highlight the tools presently in use to monitor or research these effects, identify knowledge gaps, as well as future research needs and strategies. A better understanding of the effects of diverse installations will ultimately support the expansion of the MRE industry. Furthermore, this knowledge will facilitate assessments of cumulative effects and inform marine spatial planning, supporting the implementation and management of sustainable developments in our ocean.