Patients must have capacity in order to make choices about their health or treatment. In the UK, patients over the age of 16 years are presumed to have capacity to consent to treatment unless it can be shown otherwise, and judgements about capacity must not be assumed by specific diagnoses or impairments. Capacity to consent to treatment requires that the patient must be able to comprehend and retain information given about the proposed treatment, use this information in the decision-making process and be able to communicate their decision. Capacity is not an ‘all-or-nothing’ concept, meaning that patients may have the capacity for some choices but not others. Additionally, capacity can fluctuate. Hence, assessments of capacity are decision specific and should be reviewed regularly. Doctors should be aware of factors which can enhance or diminish a patient’s capacity. A competent patient’s decision about their care or treatment must be respected regardless of whether doctors agree with it or not.