Six different capacities are reviewed: capacity to commit a crime, competency to stand trial and to assist an attorney, competencies to consent to treatment and research, capacity to express a preference in child custody disputes, and competency to testify. Each review includes a summary of relevant legal concepts and trends, an analysis of theoretically relevant psychological skills and abilities, and a review of empirical research addressing children's capacities in each area. While there is individual variation in rates of cognitive development, results suggest that particular competencies are achieved at differing ages. For instance, a 6-year-old generally is able to comprehend the notions of intent and motive and to reason about others' intentions to do harm, while the capacity to reason about the choice of a custodian requires sophisticated cognitive processes which may not be sufficiently developed before age 12 or 14. While in certain instances a 4-year-old could meet the criteria for competency to testify, such a child could not validly consent to surgery under the appreciation standard. Further, psychological factors alone are insufficient to determine if a child should be permitted to exercise his or her legal capacities in a given sphere: the relevance of competency or capacity to the legal system also depends upon policy goals and procedures that define the system. 39 notes and 65 references.