Aggression refers to goal-directed motor behavior that has a deliberate intent to harm or injure another object or person. However there are many variations of this definition. The general consensus is that aggression refers to behavior, anger, aggression, and hostility that have been used interchangeably by some researchers and clinicians, while defined distinctively different by others. There is a lack of conceptual differentiation between the terms used to represent target behaviors, which has led to confusion in differentiating between predictor and criterion measures. This lack of clarity may be representative of the theoretical overlap of concepts, or it may be that some terms represent behavioral manifestations of the higher level organizing principles represented by other terms. Lack of definitional clarity may represent disagreement among researchers in defining aggression. Through the direct and specific measurement of aggression, the greater the utilitarian value of the criterion measures. Some of the most significant differences in measurement tools involve the nature of the construct, method of data collection, utilization of participant population, and statistical implications of the possible range of scores. Use of a statistically reliable measurement instrument, appropriately chosen based on its applicability to the population being studied and the types of questions being asked, is central to effective research. Construct definition and clarification in the study of aggression are complicated by a number of factors related to choice of instrumentation and participant population. This overview of current aggressive measures is offered as an aid for selection of task-appropriate instruments to meet the needs of both clinicians and researchers. It is anticipated that this review will stimulate interest in both measurement development and concept definition and clarification. Tables, references