Biomechanics is helping us to understand all kinds of animal locomotion, from the crawling of worms and the swimming of jellyfish to the flight of birds. Much of our understanding of human movement had its origins in research on animals. Human walking is unique; no animal walks as we do. However, research on hopping kangaroos has revealed basic principles that apply as much to running as to hopping, and to humans as to animals. Early work using simple methods showed how energy is saved by the elasticity of tendons. More recent research using sonomicrography and ultrasonic imaging has shown how some leg muscles may remain almost isometric while their tendons stretch and recoil. Repeated stretching under high stresses may result in fatigue damage and overheating in leg tendons of running animals. Early research on animal swimming and flight used conventional hydrodynamics, as applied to aeroplane wings and helicopter rotors. This led to paradoxical results, failing for example to explain how some insects could generate enough aerodynamic lift to fly. This failure was due to the unsteady effects resulting from the continually changing angles and velocities of the wings. Understanding of these effects is growing rapidly, aided by digital particle image velocimetry (which reveals the patterns of flow that flying and swimming animals produce in the fluid around them) and by computational hydrodynamics. One topic that has attracted a lot of attention recently has been the relationship between the properties of muscles and their behaviour in the intact animal. Here unsteadiness is again a problem, albeit in a different way. Conventional force-length and force-velocity experiments cannot by themselves explain what happens in rapid cycles of lengthening and shortening. Work loop experiments, simulating in vitro the muscle’s in vivo behaviour, are giving increased but far from complete understanding.