SummaryMental capacity refers to a person's ability to make decisions, which may include consenting to medical treatment. People are presumed to have this capacity. Individuals may lack capacity because of an impairment or disturbance that affects the way their mind or brain works. Legal frameworks are in place for acting and making decisions on behalf of individuals who lack the mental capacity to do so for themselves, in their best interests. A case illustration is used to outline the principles involved under the Mental Capacity Act 2005 for England and Wales. We describe in detail the assessment of capacity to consent to treatment in this case and good practice in making best interests decisions in everyday clinical practice.