The Mental Capacity Act 2005 was enacted with the aim of improving the protection provided by the law to the most vulnerable people in society. The principles of the act specify to whom the act can be applied, and how to ensure these individuals are placed at the centre of decision-making about their treatment and care. It covers those people who raise objections to treatments and care proposed by health and social care professionals and, crucially, it offers protection to those who raise no objection. Appropriate application of the act should ensure all treatment and care planned for vulnerable people is in those individuals’ best interests. A best-interests decision involves considering the person’s past and current written and spoken wishes, listening carefully to those close to the individual and being aware of biases that might influence the consultation process. Nurses have an important role in this process because of their unique position in the healthcare team.