There is now considerable interest in marine renewable energy both in the UK and elsewhere in the world with the growing recognition that fossil fuel supplies are finite and further increases in carbon gas emissions will hasten climate change. A major factor in the viability of any potential wave or tidal energy device (marine energy converter (MEC)) is its reliability and availability. However, as for any new technology, this is difficult to predict in advance with confidence. This paper describes the issues that must be addressed to assess the reliability and availability of MECs. It starts with a brief overview of the characteristics of the principal types of wave and tidal energy converters and a review of earlier work on estimating reliability for wave energy converters. It is shown that MECs have many subsystems and components in common with one another and with other existing land-based and marine systems. The ways in which experience with existing systems may be adapted to predict reliability and availability for MECs are discussed and a new embryonic database for MECs is outlined. The choice of distribution for the time to failure of MEC components is discussed and an argument is advanced for the use of the log-normal distribution. The availability of MECs is affected very much by the environment in which they operate and the maintenance strategy adopted. The effects of environment and location upon maintainability and hence availability are described and how availability may be modelled is discussed. Finally the need for a framework for assessing the reliability and availability of MECs and other related issues are discussed.