The Mental Capacity Act 2005 is used to assess a person’s decision-making ability and to make a best interests decision where the individual lacks the capacity to do so. The act was considered to have the potential to maximise the autonomy of individuals with mental or cognitive impairments, but this has not been fully realised. Since the act came into force, the United Nations Convention on the Rights of Persons with Disabilities has challenged many of the principles underpinning the act. That challenge and how existing provisions in the act can be implemented to increase autonomy and control for people with cognitive or mental impairments are discussed. The principles of the convention are described and potential future amendments to the law are identified.