Webb, P. W. (1993). Swimming. Pages 47-74 in D. H. Evans, ed. The Physiology of Fishes, CRC Press, Boca Raton, Florida (USA). By definition, fish swim, but "swimming" is a loose term for a wide and complex set of adaptive movements whereby fish perform the numerous activities necessary to survive in diverse habitats. As a result, physiological studies pertinent to swimming are legion, necessitating discussion of only a few selected topics in this chapter, which focuses on the nature and properties of the propulsion system. This is comprised of a propulsor that transfers momentum from the fish to the water, thereby generating thrust, and the muscles that drive those propulsors. Neural control systems are not discussed. Swimming energetics are discussed because driving the propulsors is a major expense affecting design criteria for many other physiological systems as well as the impact of fish on their ecological resource base. In addition, the amount of energy available for swimming is often constrained by environmental factors. Scale effects are omitted.