Physicians assess the decision-making capacity of their patients at every clinical encounter. In any capacity assessment four specific abilities should be assessed: the ability to understand information about treatment; the ability to appreciate how that information applies to their situation; the ability to reason with that information; and the ability to make a choice and express it. This article explores how these abilities can be assessed through clinical interviews or formal capacity assessment tools, and highlights key situations where health care practitioners should consider performing a capacity assessment.