This report describes a naturalistic observational study concerned with the functions of aggression in children and how they change with age. Background on aggression is provided through a discussion of the problems of definition and ontogenesis, which have led to a general shortage of relevant developmental data. This study involved 102 children, 64 between the ages of 4 and 6, and 38 between 6 and 8. They were involved in six groups operating under a common program philosophy. Each aggressive act was coded as to general function: (1) Hostile, or person-directed, and (2) Instrumental, or object-directed. A finer analysis of function involved nine categories ranging from bodily injury and property destruction to rejection, derogation, and defiant non-compliance. Antecedent events were coded into three basic types: blocking, bodily contact, and derogation. Results are discussed in terms of age, sex, and race comparisons. It is concluded that the results support the hypotheses that the developmental course of human aggression can best be studied through a differentiated "functional analysis" of the problem, and that the instrumental-hostile differentation is useful in such an analysis, at least for studying early childhood. (DP)