We examined the validity of a measurement system for the study of aggression that distinguishes among four principle dimensions of aggressive behaviour: overt and relational aggression (i.e., the "whats") and instrumental and reactive aggression (i.e., the "whys"). The sample comprised 1723 adolescents (Grades 5 through 10) from Berlin, Germany. The internal validity of the measurement system was strongly supported, revealing four discrete dimensions of aggression: two overriding forms (overt and relational) and two underlying functions (instrumental and reactive). The differential and unique patterns of criterion-related validity strongly supported the distinctions among the constructs. The importance of disentangling these dimensions in understanding the development of aggressive behaviour is discussed.