Most varieties of illness range from mild to severe, from transient to long-lasting. But lack of blood test numbers and biopsy staging makes the classification of "severe and persistent illness" especially difficult in psychiatry. The reasons for needing to classify are economic. Following World War II, many Western nations experienced an economic growth and adopted a social philosophy that fostered the development of universal health and social services. Adult Severe Mental Illness is written in the context of the British National Health Service, but the questions it raises apply to psychiatry in general. Since the 1950s, economic growth has fluctuated, demands for health services have grown, a demographic shift toward an increasingly elderly population has placed unexpected strain on service use, and new technologies have not come cheap. As elsewhere, the British government has long realized that indefinite growth of universal health services is not sustainable, and numerous reforms have been instituted.