The objective of casemix development activity in the arena of mental health, as in other health care domains, is the development of a comprehensive classification of needs, activities and outcomes for all mentally ill people receiving health and/or social care. This chapter reviews work undertaken on classifications, casemix and minimum data sets applicable to service providers for the mentally ill and points the way for further work which is required if an acceptable measure of casemix usable with mental illness data is to be devised. Emphasis on the collection of diagnostic information has not served mental health services well: assigning a psychiatric diagnosis is not always easy or consistently reliable. The chapter argues that much work is necessary if real progress in understanding and measuring mental health provision is to be made. The work to be done includes the clear assignment of a diagnosis and the explicit and recordable structuring of treatments at the point of service delivery.