There is an urgent need for a rigorous science of patient and public participation, involvement and engagement (PPIE) in child and adolescent mental health research. In this Perspective we argue that current PPIE practices lack solid theoretical foundations and require systematic evaluation of their epistemic and ethical impacts. We call for the development of clear concepts of ‘lived experience’ and ‘voice’ and the establishment of a shared framework to assess the value of PPIE in advancing discovery science. We end with ten initial questions to advance open, multi-stakeholder dialogue and debate on PPIE in the field.