Generally speaking, primary prevention has been conceptualized as ontogenetically early intervention (see Cowen, 2000, for a discussion). That is, programs seeking to prevent the emergence of some problem behavior or form of psychopathology in a population typically are construed as programs that need to be provided to children as early in development as possible given the constraints imposed by their social, emotional, and cognitive capacities. From a strictly logical standpoint, this assertion makes sense especially with regard to aggressive behavior. Aggression emerges fairly early in development (Tremblay, 2000) and can lead to socially and financially costly outcomes later on (Huesmann et al., 2002; Jones et al., 2002). Thus, it seems reasonable to maintain that aggression and violence prevention programs should target children at as young an age as possible. However, primary prevention of aggression can occur throughout childhood and adolescence given the developmental underpinnings and variants of this behavior. This chapter discusses key developmental issues and concerns in the primary prevention of aggression in school-age children and adolescents.