Many aquatic plants and animals spend part of their lives anchored to the substratum as water flows by. There are a number of mechanisms by which such sessile organisms can affect the magnitude of the flow-induced forces they encounter, as well as the distribution and magnitude of the mechanical stresses in their bodies produced by those forces. Furthermore, the mechanical properties of the skeletal tissues of such organisms affect how much they deform and whether or not they will break in response to flow-induced stresses. There are different mechanisms by which organisms can withstand the water flow characterizing a particular type of habitat. Biomechanics is a useful tool for studying how the performance of organisms depends on their structure. Biomechanical studies should be accompanied by knowledge of the natural history and ecology of the organisms in question if they are to lead to insights about how organisms work.