Psychiatry is the branch of medicine that deals with mental, emotional, and behavioural disorders. In psychiatry, as in other branches of medicine, classifi cation of disease is useful for the following: ? Informing clinical practice. By placing a given patient?s disorder into a recognized diagnostic category, the clinician is able to make an appropriate choice of treatment, and judge the probable future outcome. Using a classifi cation system in this way does not, of course, remove the need to consider and respect those features that are unique to each individual case. ? Communication between professionals. A universally understood classifi cation system permits effi cient communication, whether in everyday clinical practice when colleagues are discussing a case, or in the national and international literature. ? A basis for research. Research workers require a classifi cation system in order to investigate the causes, clinical features, natural history, and response to treatment of the various psychiatric disorders. ? Service planning. The type of treatment services required in a given area will depend on the frequency of different disorders within the local population.