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 fourth grade. In addition to ratings of children's impulsivity, the analyses included measures of motivation, cognitive skills, socio-emotional 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: first, a group with low impulsivity; second, a group with decreasing impulsivity; third, a group with moderate impulsivity; and, fourth, 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 socio-emotional 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 socio-emotional adjustment and teacher-student relationships in a meaningful way, the findings highlight the importance of behavioral regulation skills in the classroom.Keywords: behavioral regulation, developmental trajectories, early school years, impulsive behavior, self-regulation, socio-emotional adjustment, teacher-student relationships Impulsivity Trajectories from Kindergarten to Grade 4 3 Identifying Finnish Children's Impulsivity Trajectories from Kindergarten to Grade 4:Associations with Academic and Socio-Emotional Development It is widely acknowledged that self-regulation as manifested in children's ability to monitor, modulate, and direct their cognitive functions, attention, emotions, and behavior (e.g., Berger, Kofman, Livneh, & Henik, 2007;McClelland & Cameron, 2012) is critical for children's school readiness and successful adjustment (e.g., McClelland et al., 2007;Ponitz, McClelland, Matthews, & Morrison, 2009). Children's ability to control impulses and, thus, regulate their behavior in the classroom context contributes to how they benefit from that environment with respect to learning and adjustment outcomes (Morrison, Ponitz, & McClelland, 2010). Behavioral regulation has been shown to be an important predictor of both academic (McClelland, Acock, & Morrison, 2006;Valiente, Lemery-Chalfant, Swanson, & Reiser, 2008) and social functioning (Eisenberg et al., 1995;Fabes et al., 1999), and to have associations with teacher-student relationship quality (Henricsson & Rydell, 2004;Ladd, Birch, & Buhs, 1999). Ability to regulate impulsive behavior as a component of selfregulation is highly relevant in the classroom context where children are expected to be able to sit still, to wait for their turn, to follow instructions, and to think before acting. Yet, the de...