Consultation-liaison (CL) psychiatry is psychiatry for the medically ill. This review describes the evolution of CL psychiatry from its origins in early 20th century general hospitals to the present era. The differences between CL psychiatry and general psychiatry are emphasized. The CL psychiatrist works in a multi-specialty and interdisciplinary environment whereas the general psychiatrist may not. Because of the CL psychiatrist's experience with comorbid psychiatric and physical illness, he/she is skilled in the management of psychiatric comorbidity in the medically ill. Healthcare systems now require integrated models of service delivery in resource-sensitive environments. The CL psychiatrist is qualified to advise fellow physicians and other healthcare practitioners in how to manage psychiatric illness. CL psychiatrists have clinical, educational, administrative and research roles that are valuable in the delivery of comprehensive medical care in integrated, outcome-oriented systems.