Tidal stream devices are a new technology for extracting renewable energy from the sea. Various tidal stream device models have been proposed and, if they are installed at chosen high tidal stream velocity sites, they may face extreme climatic, current, and wave load conditions. As they contain complex mechanical, electrical, control, and structural systems, reliability and survivability will be a challenge. Data on their reliability have been scarce, as only a few prototypes have been built and operated. However, reliability prediction of new devices could minimize risk in prototype work. A practicable tidal stream device reliability prediction method could assist the development of cost-effective and viable future options. The present study proposes such a method and derives system reliability models for four generic-design, horizontal-axis, tidal stream devices, all rated 1-2 MW. Historical reliability data from similarly rated wind turbines and other relevant marine databases were used to populate the devised reliability models. The work shows that tidal stream devices can expect to have a lower reliability than wind turbines of comparable size and that failure rates increase with complexity. The work also shows that with these predictions, few devices can expect to survive more than a year in the water. This suggests that either predicted failure rates must be reduced dramatically or that methods for raising reliability-by the use of twin axes or improving maintenance access by unmooring or the use of a seabed pile and turbine raising-will be needed to achieve better survivor rates. The purpose of this work is not to predict definitive individual device failure rates but to provide a comparison between the reliabilities of a number of different device concepts.@en