When a patient appears to have a mental disorder, doubts may arise about his or her decision-making capacity. Health professionals must then assess the patient's capacity in order to make sure of the validity of his or her consent or refusal. Incapacity has indeed legal consequences, as law provides for the appointment of a surrogate decision-maker in case of incapacity. With Belgian law as a point of departure, this contribution is aiming at identifying the role of law in capacity assessment itself, prior to the decision about (in)capacity. In order to protect the patient's rights and to support the task of those carrying out the assessment, law should provide for a global definition of decision-making capacity and for a frame-procedure guiding this assessment. In my opinion, it is possible for law to contribute to the complex task of capacity assessment without interfering embarrassingly with healthcare practice.