The dramatic long-distance flights of butterflies and other large insects, occurring near the ground, have long been regarded as migratory. In contrast, high-altitude wind-borne movements of small insects have often been viewed differently, as uncontrolled or even accidental displacements. This chapter shows how an individual-based behavioral definition provides a unifying framework for these, and other modes of migration in insects and other terrestrial arthropods, and how it can distinguish migration from other types of movement. The chapter highlights some remarkable behavioral phenomena revealed by radar, including sophisticated flight orientations shown by high-flying migrants. Migration behavior is always supported by a suite of morphological, physiological and life-history traits—together forming a ‘migration syndrome’, itself one interacting component of a ‘migration system’. These traits steer the migrants along a ‘population pathway’ through space and time, while natural selection acts contemporaneously, continually modifying behavior and other aspects of the syndrome.