This longitudinal study investigated the contribution of achievement goals and academic self-concept for the prediction of unexpected academic achievement (i.e., achievement that is higher or lower than expected with respect to students' cognitive ability) in general and when comparing groups of extreme over- and underachievers. Our sample comprised 769 students (50.78% female, 160 underachievers, 176 overachievers) in the highest track of the German secondary school system who were assessed three times (in grade 5, 6, and 8). Controlling for students' cognitive ability, results supported the reciprocal effects model for positive developmental relations between academic self-concept and achievement in the overall sample. Furthermore, academic self-concept and performance goals showed significant reciprocal relations. For achievement goals, only mastery goals showed positive reciprocal relationships with academic achievement, whereas performance goals (approach and avoidance) showed no significant relations. However, for underachieving students, academic self-concept as well as mastery goals were unrelated to later achievement, and for overachieving students, both constructs showed positive relations with later achievement. Findings indicate that interventions aimed at positive goal development might also produce positive and lasting impacts on academic self-concept and--mediated by self-concept--on academic achievement. However, this does not seem to apply to underachieving students.