Aggression is a multifaceted phenomenon, with different concepts under the same term. A lack of agreement in the attempts to categorize its different types explains the overabundance of classifications found in the literature, with a consequent poor conceptual clarity. Some authors use the form or mode of the aggressive act as a criterion of classification, and others prefer to focus aggression on its function, purpose, or goal. This manuscript reviews a variety of functional classifications of aggressive behavior, analyzing similarities and differences among them, in an attempt to clarify the nature of diverse types of aggression. A literature review compares various dichotomies of aggression, presenting comparisons between instrumental and hostile aggression, between proactive and reactive aggression, and between premeditative and impulsive aggression. An analysis of empirical data from some self-report instruments applied in our and in other laboratories shows high correlations between hostile, reactive and impulsive aggression, on one side, and between instrumental, proactive and premeditate aggression, on the other. This supports the convenience of unifying some measurement instruments in order to offer a more conceptual clarity and parsimony in the categorization of aggression.