Common law dictates that individuals possess autonomy and self-determination, which encompass the right to accept or refuse medical treatment. Management of medical treatment can be complicated in situations when the ability of the patient to make reasonable decisions is called into question. Our legal system endorses the principle that all persons are competent to make reasoned decisions unless demonstrated to be otherwise. This review will discuss the standards upon which capacity and competency assessments are made. Practical suggestions are offered for clinicians to employ in patient interviews conducted to assess capacity. Issues related to advance directives, surrogate decision making, guardianship, and implied consent are also discussed. The role psychiatric consultants take in capacity assessment can assist the primary care physician confronting the complexities encountered when attempting to treat the incapacitated or incompetent patient.