With an increasing requirement to lower the costs of delivered renewable energy, the maintenance costs for marine renewable energy (MRE), due to accessibility issues, are an obvious focal point. In particular, condition-based maintenance and prognostics can help to optimise maintenance activities and forewarn of impending maintenance requirements, mindful of the constrained access to MRE systems due to limited weather windows of sufficient duration. This paper focusses on offshore wind, tidal flow and wave energy as target MRE domains and provides a comprehensive review of condition-based maintenance methodologies currently employed in MRE systems. While, of the three energy domains, offshore wind is the more mature, giving the opportunity to propagate such methods to the less mature areas of tidal and wave, there are also many components and challenges which are common to all three domains, e.g. generator systems and grid interface.