Offshore Renewable Energy (ORE), comprising marine (wave and tidal energy), and offshore wind, has the potential to supply large amounts of ‘green’ sustainable energy, reducing CO2 emissions. The main obstacles to deployment so far are technical challenges and cost. However, there are also concerns about how harnessing offshore energy can affect the local habitats and marine life, as well as introducing far-field and long-term changes in the physical environment of the sea, which may combine with climate change in unforeseen ways to affect marine ecosystems. The precautionary principle, combined with the requirement for monitoring, introduces obstacles (and costs) which have so far prevented the deployment of offshore renewable energy on a large scale. Here we discuss the physical changes that may occur and the impacts these may have on habitats, species and ecosystems. We explore the possible environmental impacts of offshore wind and marine energy deployment and the options for mitigation of these. This information can assist planners, regulators and developers of offshore energy systems. Some examples of existing and proposed deployments are provided (mainly focusing on the UK), in order to illustrate discussion of the environmental issues. We identify the need for better understanding of the environmental impacts at a population and ecosystem level and identify a way forward to improve the environmental consenting process.