This Practice Note explains that the Mental Capacity Act 2005 (MCA 2005) creates a presumption of capacity, in the absence of proof to the contrary, for adults to make informed decisions. It sets out the principles that govern the determination of capacity and a person’s ability to rationalise a decision under MCA 2005, and discusses the Code of Practice for those working with or caring for those lacking capacity. It covers the MCA 2005 definition of the best interests of a person who lacks capacity, and the protection available for those responsible for the care of those who lack capacity. It explains that capacity to make decisions are both time and decision specific and the factors to consider when determining capacity to decide where to live and what care to receive.