The negative impact of alcohol or drug misuse on the family has been well documented. However, the predominantly individualistic approach to treatment and policy has led to lack of accurate calculations of the numbers of affected family members and the associated costs to families and society of their suffering, the stress they experience and the care they provide to the substance misuser. For the practice and policy agendas to continue to evolve, such estimates are vital. This article summarizes what is currently known about the numbers of affected family members and the associated costs to families and society. The main findings from the first piece of research undertaken in the UK to model the numbers of adult family members of illegal drug misusers and associated costs are then presented. Finally, this article discusses the strengths and limitations of such models and the implications for both service development and policy.