The extraction of marine energy, through either tidal or wave array operation, will clearly influence the hydrodynamics of a region. Although the influence on tidal currents and wave properties is likely to be very small for most extraction scenarios, the influence on bed shear stress is likely to be greater, because bed shear stress is quadratically related to tidal currents and wave orbital velocities. Further, the transport of sediments is a function of tidal current and wave orbital velocity cubed. Therefore, even small modifications to the flow field through tidal or wave array operation could lead to significant impacts on regional sediment dynamics. In this chapter, after providing an introduction to sediment dynamics in the marine environment, we explore the impact of tidal energy devices/arrays on regional sediment dynamics, with a particular emphasis on offshore sand banks—important sedimentary systems that protect our coastlines from the full impact of storm waves. Next, we discuss how generating electricity from waves could influence nearshore sediment processes, such as beach erosion or replenishment, over a range of timescales. To assess the magnitude of impacts on sedimentary systems, it is essential to consider the scale of the impact in relation to the range of natural variability. We suggest ways in which impacts can be assessed using numerical models, tuned by in situ measurements, that quantify variability over a range of timescales from individual storm events and lunar cycles to seasonal and interannual periods. We also discuss the sedimentary processes associated with tidal lagoons, such as scour and sediment drift outside a lagoon and sediment accretion inside a lagoon.