Why should developmental and life-course criminologists explore prosocial behaviour? One might argue that they already do, and indeed, in this chapter, I will provide evidence of where prosocial behaviour features in developmental and life-course criminology (DLC) literature. Where prosocial behaviour is featured in DLC literature, the dominant assumption is that it is the absence or opposite of antisocial behaviour. There are numerous examples of this assumption: Where an intervention results in reduced antisocial behaviour it is assumed that prosocial behaviour increases; where there is the absence of adolescent antisocial behaviour it is assumed that these adolescents are prosocial; and where there is desistance from antisocial behaviour it is assumed that prosocial behaviour replaces the antisocial behaviour. However, there has been a lack of research that confirms these assumptions. In this chapter, I will highlight how DLC scholars can fill this research gap by drawing on their knowledge of the development of antisocial behaviour and of longitudinal research methods to better understand the interplay of prosocial and antisocial behaviour. I argue that expanding on the evidence in this way will provide a stronger theoretical and empirical foundation for the preference for strength-based developmental prevention interventions, especially in the social services sector.