Psychiatrists working in consultation-liason services are often called upon to assess the competency of patients who are under the primary care of other physicians. Despite the frequency with which this sort of problem is encountered, no clinically useful guidelines seem to exist to aid the clinician in making such an assessment. In fact, a number of very different sorts of "tests" for competency to consent to medical care seem to exist and are used in practice. This essay attempts to make some headway in defining a general set of clinically useful criteria for the determination of competency to consent to medical care. This is done by looking at the central elements of informed consent itself and then asking what capabilities are required by anyone seeking to give consent to medical care.