Research on aggressive behavior currently suffers from problems in measurement. It is contended that two fundamental issues underlie these limitations: (1) operational definitions of aggression and its various subtypes lack sufficient conceptual clarity and (2) many assessment instruments used in the study of aggression do not appropriately correspond to precise definitions of aggression or its subtypes. As a first step toward resolving these limitations, a new taxonomic system is proposed. The development of this classification system was based on hybrid definition of aggressive behavior, clear distinctions between subtypes of aggressive behavior, and pertinent empirical findings. This proposed approach to the categorization of aggressive acts will bring greater conceptual clarity to the assessment of aggression and advance empirical, theoretical, diagnostic, and clinical work in this complex area. Aggressive behavior is a multifaceted construct that may be expressed behaviorally in a myriad of ways. Because of this, the development of assessment instruments that aim to precisely measure this wide range of behaviors is a challenging task. These challenges cause current research on aggressive behavior to suffer from limitations in measurement. For this reason, the aim of this paper is to facilitate the resolution of the criterion problem. It reviews the manner in which operational definitions of aggression are frequently misinterpreted, as well as the lack of precision involved in the categorization of subtypes of aggression. It then illustrates a disconnect between operational definitions of aggressive behavior and instruments designed to assess it by reviewing the limitations in the construct validity. It then contends that the resolution of the criterion problem requires the endorsement of a hybrid operational definition of aggressive behavior and the creation of a new classification system. The proposed classification system is a modification of a taxonomic system. Table, references