The goal of this study was to test if both effortful control (EC) and impulsivity, a reactive index of temperament, uniquely predict adolescents' academic achievement, concurrently and longitudinally (Time 1: "N" = 168, X-bar[subscript age] = 12 years). At Time 1, parents and teachers reported on students' EC and impulsivity. At both time points, spaced 2 years apart, parents and teachers reported on students' achievement. In a concurrent regression, both EC and impulsivity were positively related to achievement. At T1, there was evidence of a nonlinear relation between impulsivity and achievement, and the shape of the quadratic was dependent on if EC was simultaneously considered. Results from a longitudinal analysis demonstrated that although parent-reported impulsivity was generally negatively correlated with achievement, EC, but not impulsivity, was prospectively, uniquely related to achievement. The discussion highlights the value of considering adolescents' EC and impulsivity in models of school success.