This article considers the requirements set out in the Mental Capacity Act 2005 for valid advance decisions. The Act recognizes that an adult with capacity may refuse treatment, including life-sustaining treatment, in advance of losing capacity. If that advance decision is valid and applicable, it will bind health-care professionals, taking effect as if the patient had contemporaneously refused the treatment. However, in cases where the advance decision does not relate to treatment for a progressive disease, it will be extremely difficult for the patient to meet the dual specificity requirement - specifying the treatment to be refused and the circumstances in which that refusal should operate. Moreover, while a patient may explicitly revoke an advance decision while she retains the capacity to do so, the continuing validity of an advance decision may be called into question by the patient implicitly revoking her advance refusal or by a change of circumstance. This article concludes that the key to enabling patients to exercise precedent autonomy will be full and frank discussion of the scope and intentions underlying advance decisions between patients and their health-care professionals.