In this lecture, the author proposes to explore the concept of decision-making capacity (DMC), how it impacts various fields including medicine, law, and psychology and how challenging can be its assessment and consequences. DMC represents an individual’s ability to make choices autonomously, being a precondition to any informed consent.There is no single diagnosis that automatically concludes for incapacity or waives its assessment. On the other hand, DMC can only be assessed in relation to a particular decision and its results are not necessarily stable over time. In some cases, it can even be improved with the appropriate approach. It depends on various factors. Cognition is one of the most relevant, whereby any illness or treatment that affects cognition may be associated with reduced or compromised capacity, requiring a formal assessment. Other factors can influence the decision-making process, without compromising it, like individual’s level of education, personality traits, emotional states/coping mechanisms or circumstantial or even cultural factors. Thus, in order to protect vulnerable patients, it´s assessment brings to light the tension between the respect for patients’ autonomy (acting in a patient best interest) and providing the best treatment available (acting in medical best interest), that may not be coincident. Even though every doctor should be able to carry out this evaluation, an expert in capacity assessment should be involved in case of suspected mental or neurological illness that can impair the ability to consent. Is also important to validate decisions with high risks, namely when there is a huge difference between risks and benefits or when the patient dissents for the recommended treatment. This can be a complex and highly time-consuming activity.