Childhood psychological morbidity places both short-term and long-term costs on families, health services, social services, voluntary organizations, and society as a whole. Internationally, there have been few economic evaluations alongside clinical trials of population-based interventions to improve child mental well-being. This paper sets out a number of trade-offs relevant to the evaluation of the costs and benefits of multisectoral interventions to improve child mental well-being. These trade-offs have relevance for those who fund research, those who conduct research, and those who make policy decisions. The paper sets out a conceptual model for the economic evaluation of population-based interventions to improve child mental health. Finally, the paper argues that with the advent of "joined up" public policy and the breakdown of traditional ring-fenced budgets for health and social care, there is now, more than ever before, an opportunity for the research community to generate guidance for multisectoral collaborative interventions to promote the mental well-being of children and their families.