The present research examined the joint effects of social, dimensional, and temporal comparisons on students' academic self-concepts and parents' beliefs about their children's competencies during students' transition phase from elementary school to junior high school. To this end, we tested the newly developed 2I/E model using longitudinal data including 2,417 students between Grades 6 and 7 and 1,846 parents. We found strong social comparison effects and moderate dimensional and temporal comparison effects on students' self-concepts. In line with our assumption that students should be particularly sensitive to changes in their achievements during phases of school transition, the temporal comparison effects were stronger than in any prior study of the 2I/E model. Parents' ratings of their children's competencies were also predicted by social, dimensional, and temporal comparisons. However, whereas the social comparison effects on students' and parents' ratings were equally strong, the dimensional and temporal comparison effects were stronger on students' ratings. We finally tested how social, dimensional, and temporal comparisons affected changes in students' self-concepts and parents' competence beliefs over 1 year. Although the effects of all 3 comparisons were still significant, we found drastic changes in their strengths, with the strongest effects on changes in competence beliefs occurring for temporal comparisons. Overall, our study provides a variety of new insights into the impact of comparison processes on the formation of competence beliefs. In particular, it emphasizes the role of temporal comparisons. Theoretical and practical implications are discussed.