This chapter considers the importance of royal heirs in the process of militarising the monarchies of nineteenth-century Europe. It gauges the significance of the crowns’ military performance for their attempts to generate loyalty and popular endorsement. Royal heirs, and crown princes in particular, had a significant part to play within this context – if only as military actors, but even more so if they could demonstrate real military achievements. The chapter culminates in an assessment of the experiences and contributions of three royal heirs who assumed military roles during the First World War.