In order to clarify the complicated relationship between the onset of puberty and the transition from elementary to junior high school, a controlled study was designed that compared the transition of grade 5 elementary school students to grade 6 in an elementary school with the transition of grade 6 elementary school students to grade 7 in a junior high school. Two sets of measures are reported. One set consists of children's perceptions of their competence in three domains that are important for their adjustment in school (i.e., their perceptions of their cognitive competence in academic matters, their social competence in peer relationships, and their physical competence in sports and outdoor games) plus their perceptions of their general competence, representing a measure of self-esteem. The second set of measures reported consists of teachers' ratings of the children's competence in the same areas. The scales were administered three times: once prior to transition, toward the end of the school year, and twice after transition -- about 10 weeks into the next school year and again toward the end of that school year. Findings reported in this paper focus on 226 children in the longitudinal sample for whom data was available at all three times of measurement. Discussion centers first on the effects of transition on this entire group of children and then on the effects of physical maturity and transition on a subsample of these children. (Author/RH)