Wave and tidal energy devices are subjected to normal everyday loadings and abnormal extreme loadings. Extreme loadings are severe and less frequent. The repetitive loadings arising from wave-device interaction or current-blade-structure interaction are lower and occur very frequently in normal operation. Economic designs that will survive have to withstand, without structural failure, a combination of these types of loading over the design life of the device and its subsystems. Cumulative fatigue damage in the wave or turbulent-current environment could occur earlier than anticipated in the life of wave or tidal current technologies and needs to be better understood to predict wear-out or failure and ensure designs are robust without entailing excessive cost. This work will explore numerically through computer modelling, and physically through preliminary model- and sea-testing, the interaction of tidal and wave devices with their sea that surrounds them, one another, their moorings and the electricity network to understand the cyclic and irregular forces acting and the structural loadings arising, ultimately aiming to reduce fatigue effects and increase reliability.