There is an ongoing development of offshore renewable energy projects worldwide. Marine wind power technology is the most advanced and tidal and wave power projects are creating increasing interest. Marine renewable energy projects to date have been focused in northern Europe, yet developments are also planned and underway in other parts of the world. Whilst these offshore renewable energy developments are typically characterised as environmentally desirable, there are some associated adverse impacts that deserve careful consideration. However, the renewable energy industry is in some ways still in its infancy and, as such, not all of its impacts are clear or fully assessed. This paper outlines the global development of this sector, identifies species of particular concern, outlines some of the possible effects from renewable energy developments on cetaceans and identifies research and monitoring needs to address important knowledge gaps.