Capacity evaluations for medical aid in dying are required only if a physician who has received a request for a lethal prescription suspects a patient's judgment may be impaired. • Although no empirically based instruments or protocols are available to evaluate capacity in the context of medical aid in dying, clinicians can reasonably adapt instruments used in other capacity contexts and can rely on guidelines constructed recently by state psychological associations and hospitals.• The ultimate question for the mental health professional is: Does the patient have the capacity to request a prescription to take by themselves to end their life?• As in other contexts, capacity is based on four standards the patient must demonstrate: understanding, appreciation, reasoning, and the ability to communicate a choice.• Data for evaluating capacity can come from multiple sources, including the patient, care partners, medical providers, medical records, and previous expressions of the patient's values and preferences (e.g., advance directives).