A critical event in sexual reproduction is location or recruitment of a mate. In a number of insect groups, the necessary movements in time and space are often mediated by pheromones. One sex may recruit the other, or both sexes may be attracted to the chemical emitters. Aggregation may be viewed as the end result of movement reactions that reduce the distance between individuals in their environment. Such clustering may be brought about by a combination of attraction and arrestment, which are themselves not orientation mechanisms but rather end results, i.e., displacements, caused by movement reactions (Kennedy, 1978). For sex- and aggregation-pheromone communication, we define attraction as the net displacement of one individual toward the chemical source. Conversely, arrestment is the lack of net displacement toward or away from the chemical source. Both displacement phenomena may be viewed as part of a continuum caused by pheromone mediation of quite disparate movement reactions, such as orthokinesis, klinotaxis and anemotaxis (see Bell, Chapter 4 and Carde, Chapter 5). That attraction and arrestment are only outcomes, not mechanisms, does not diminish the heuristic value of these terms; they are a capsule summary of the change in spacing between an individual and the chemical source. To an organism responding to sex pheromone, proximate cues and orientation mechanisms notwithstanding, such outcomes are the result of evolutionary selection.