This thesis explored prosocial behaviour from a developmental and life-course perspective. In doing so, I fill two gaps in understanding in the role of prosocial behaviour in the deceleration of conduct problem behaviour. First, I explore how a developmental and life-course understanding of prosocial behaviour may shed light on whether prosocial behaviour is a facilitator or measurable outcome of the deceleration of conduct problem behaviour. Second, I explore how the social support perceptions of adolescents are related to their prosocial and conduct problem behaviour. Addressing a gap in knowledge on whether prosocial and conduct problem behaviour are driven by the same underlying factors. Ultimately, this thesis answers the overarching question––why should developmental and life course criminologists explore prosocial behaviour?