Organisms move, and their movement can take place by walking, swimming, or flying; via transport by another organism (phoresy); or by a vehicle such as wind or current (Dingle 1996). The functions of movement include finding food or mates, escaping from predators or deteriorating habitats, the avoidance of inbreeding, and the invasion and colonization of new areas. Virtually all life functions require at least some movement, so it is hardly surprising that organisms have evolved a number of structures, devices, and behaviors to facilitate it. The behavior of individuals while moving and the way this behavior is incorporated into life histories form one part of this chapter. This discussion focuses on the action of selection on the evolution of individual behavior, on how specific kinds of movement can be identified from the underlying behavior and physiology, and on the functions of the various movement behaviors. The other major part of our discussion focuses on the consequences of movement behaviors for the ecology and dynamics of populations. The pathways of the moving individuals within it can result in quite different outcomes for a population. First, movements may disperse the members of the population and increase the mean distances among them. The separation may be a result of paths more-or- less randomly chosen by organisms as they seek resources, or it may be a consequence of organisms avoiding one another. In contrast to dispersing them, movement may also bring individuals together either because they clump or congregate in the same habitat patch or because they actively aggregate through mutual attraction. Clumping can also lead to aggregation and mutually attracting social interactions. A classic example is the gregarious (aggregating) phase of the desert locust (Schistocerca gregaria), in which huge swarms of many millions of individuals first congregate in suitable habitats and then develop and retain cohesion based on mutual attraction. The foraging swarms make the locust a devastating agricultural pest over much of Africa and the Middle East (Farrow 1990; Dingle 1996). It is the aggregation of locusts that makes them such destructive pests; they would be far less harmful if the populations dispersed.