The pace of development for wave and tidal energy projects worldwide continues to be hindered by uncertainty surrounding potential environmental effects of the devices and the balance of system. To respond to this uncertainty the Ocean Energy Systems (OES) international agreement developed a collaborative initiative (Annex IV). Over an initial three-year period (2010-2012) Annex IV collected metadata on environmental effects around marine energy projects, as well as ongoing research into interactions of wave and tidal devices and the marine environment. Housed on the US-based Tethys online knowledge base the Annex IV metadata was used to investigate high priority interactions. The work of Annex IV was guided by expert input at two workshops held in Dublin (2010 and 2012). The three priority environmental interactions are documented in a report available from OES and on Tethys: 1. The interaction of aquatic animals with turbine blades; 2. The effects of underwater noise from tidal and wave devices on marine animals; and 3. The effects of energy removal on physical systems. Each priority interaction (or “case study”) examined published literature, compliance and investigative reports, and information gathered directly from device developers and researchers. This information was used to reach preliminary conclusions on the importance of each interaction to the environment; assess the level of certainty surrounding each interaction; and highlight key research gaps that hinder a deeper understanding of the interaction.