Early aggression is a problem in its own right and a risk factor for further developmental problems. Although both effortful control and social information processing (SIP) skills are negatively associated with aggression and are targeted by aggression prevention programs, little is known about the relation between them or about their joint relation with aggression. Further, effortful control is often measured poorly in aggression prevention studies. This dissertation reports findings from three studies of effortful control, SIP, and aggression. Data were collected from 691 boys and girls at 10 North Carolina schools during the third and fourth grades (2004-2006). In Study 1, exploratory and confirmatory factor analyses were used with third grade fall data to develop an effortful control scale. In Study 2, generalized estimating equations were used to assess whether, within each grade, SIP variables (measured in winter) mediated the relation between effortful control (fall) and aggression (spring). Similarly, Study 3 assessed whether fourth-grade effortful control and SIP variables (fall and winter, respectively) mediated the effect of the Competence Support Program, a classroom management and social skills training intervention, on aggression. Study 1 resulted in a second-order model (X[superscript 2]/df = 6.41, RMSEA = 0.089 [90% CI: 0.077 to 0.100], CFI = IFI = 0.957), with effortful control explaining the correlations among three factors: inhibitory control (alpha = 0.843), attention control problems (alpha = 0.876), and impulsivity (alpha = 0.885). In Study 2, unadjusted results showed significant indirect effects of effortful control on aggression, mediated by hostile attribution (fourth grade only), goal formulation, and response decision, but not encoding. Adjusted results showed no mediation effects. Study 3 found no effect of the Competence Support Program on mediators or on aggression. Based on a strong theoretical framework and using appropriate statistical methods, Study 1 developed a reliable and valid teacher-reported measure of effortful control; Studies 2 and 3 provided modest support for the theory underlying many aggression prevention programs. Together, the studies resulted in recommendations for improved measurement of effortful control, SIP, and aggression; better use of longitudinal mediation models to assess the relations among them; and improved research on aggression prevention. These studies belong to a body of research that is incrementally improving the prevention of childhood aggression. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]