Marine reserves are places where wildlife and habitats are protected from extractive and depositional uses of the sea. Although considered to be the pinnacle in marine conservation, many permit non-consumptive activities with little or no regulation. This paper examines the potential impacts of 16 non-consumptive activities including scuba diving, sailing, scientific research and motor boating, and how they might compromise the conservation objectives of marine reserves. Examination of 91 marine reserves from 36 countries found little agreement or consistency in what non-consumptive activities are permitted in marine reserves and how they are regulated. The two most common activities allowed without regulation were swimming (mentioned in 80% of marine reserves and allowed in 63% of these) and kayaking (mentioned in 85%, allowed in 53%). Scuba diving was mentioned in 91% and allowed without regulation in 41%. A risk score for the likely level of threat to wildlife and/or habitats that each activity could produce was then assigned based on effects reported in the literature. The risk analysis suggests that motor boating and activities which include or require it have a high potential to negatively impact wildlife and habitats if inadequately managed. Hence protection against extractive or depositional activities alone is insufficient to secure the high standard of protection usually assumed in marine reserves. For this to be achieved activities typically considered as benign must receive appropriate management, especially with increasing recreational use.