The assessment of mental capacity is a skill that every GP uses in all consultations. The majority of patients have mental capacity to make treatment decisions. Capacity is presumed until proven to be lacking. However, patients can lose their ability to make treatment decisions for a range of reasons, and decisions may need to be made on their behalf. The loss of capacity may be permanent or temporary, and these clinical decisions may ultimately be a matter of life or death. Capacity is considered situation-specific, and thus should be judged separately for each decision faced by the patient. This article gives an overview of the main pieces of legislation in the different legislative areas of the UK: England and Wales, Scotland, and Northern Ireland; and assists the GP in navigating the practicalities of applying the legislation to clinical cases to allay anxieties around working in this area.