To provide a comprehensive service for children and young people with mental health problems, commissioners require, atthe least, three set of data: information on the needs of the population; knowledge of the provider units available and the service components they offer; and financial information to assess the costs of meeting the population needs. Data such as these are vital to ensure that the services purchased do meet the needs of the population and that the desired balance between expenditure and the budget is attained. This chapter focuses on the last two of these desiderata