Movement is, for fish, quite a challenge, because water is 800 times denser than air. The technical solution to this problem has, in general, been the development of a streamlined body and powerful swimming muscles known as myotomes. In the majority of fish species, such muscles account for over half of the body weight.

The most common mode of swimming among fish is through rapid movements of the tail and body. Certain species, however, have specially modified fins and utilize these in place of body and tail oscillations. In some, pectoral fins are oarlike in appearance and function; in others, such as the seahorses and pipefish, fluttering movements of the dorsal and anal fins propel the fish about.

A fish's buoyancy also affects its swimming ability. Many fish attain neutral buoyancy by virtue of their gas-filled swim bladders. Because the depth at which the fish is swimming affects buoyancy, those species with swim bladders must deflate or inflate the organ or produce gas when they ascend to the surface or dive. Bottom-dwelling fish are largely untroubled by questions of buoyancy and often do not possess swim bladders.