Mental competence describes a person’s ability to understand, retain and use information to make decisions, and to communicate those decisions, at the time they need to be made. It is central to assessing capacity to consent to medical treatment, make a will, enter contracts, conduct litigation and manage property and affairs. In England and Wales, the concept is largely reflected in the Mental Capacity Act 2005, which sets out a decision-specific, time-specific test of capacity. Northern Ireland has similar principles in the Mental Capacity Act (Northern Ireland) 2016. In Ireland, the Assisted Decision-Making (Capacity) Act 2015 adopts a functional approach to capacity. In Scotland, mental capacity is governed primarily by the Adults with Incapacity (Scotland) Act 2000. “Mental competence” itself is often used as a descriptive, non-technical term rather than a defined statutory expression, but it overlaps with “mental capacity” and “capacity to make decisions” in legislation and case law. For legal practitioners, determining mental competence typically involves medical and sometimes psychiatric evidence, is decision- and context-specific, and has significant consequences for the validity of transactions, informed consent, advance decisions and the appointment or exercise of powers by attorneys, deputies or guardians.