Psychiatry is a rich and diverse medical specialty concerned with abnormalities of the mind, namely abnormal emotions and thoughts, which may result in abnormal behaviour. Psychiatric illness may have social, psychological and biological causes. The symptoms and signs of mental disorder are known as psychopathology. Doctors analyse a patient's psychopathology using the psychiatric history and the mental state examination. Psychiatrists use four methods of assessment: the psychiatric history, the mental state examination, a physical examination and relevant medical, social and psychological investigations. Information from these assessment methods is built into a differential diagnosis from which a management plan is formulated. The assessment is a holistic one in that it incorporates physical, social and psychological components. The chapter discusses four case histories and demonstrates that mental illness can occur at any age and that it can have a rich variety of presentations.