With an emphasis on improving the relationship between medicine and psychiatry, psychiatrists have been under pressure to prove themselves as members of a medical discipline distinct from nonmedical psychology and psychiatric social work. Psychiatry, as a branch of medicine, has been described as searching for an identity of its own as compared to other nonmedical mental health professions. The fact, that psychiatric patients can also present to the medical or surgical units due to antecedent physical illnesses and may need additional care by psychiatrists is being recognized. This chapter covers the history of evolution of concept, basic concepts, reasons of consultation, consultation versus liaison psychiatrist, roles of a consultation-liaison psychiatrist, and assessment and formulation.