Research Findings: The purpose of this study was to identify the developmental trajectories of impulsive behavior among 378 Finnish children who were followed from kindergarten to 4th grade. In addition to ratings of children's impulsivity, the analyses included measures of motivation, cognitive skills, socioemotional adjustment, and teacher-student relationship. Four latent groups were identified that differed in the level and change of the children's impulsive behavior across time: (a) a group with low impulsivity, (b) a group with decreasing impulsivity, (c) a group with moderate impulsivity, and (d) a small group with a contradictory trajectory showing an upward trend in impulsivity. The decreasing group showed compromised behavioral regulation in kindergarten but not thereafter, and it was the poorest performing group in reading. Both the moderate and decreasing groups received negative ratings from their teachers with respect to socioemotional adjustment and relationships with the teacher. The moderate group predominantly included boys, whereas the low impulsivity group had a higher ratio of girls. Practice or Policy: By linking the different trajectories of impulse control development to children's socioemotional adjustment and teacher-student relationships in a meaningful way, the findings highlight the importance of behavioral regulation skills in the classroom.