Withdrawal and limitation of life support in the intensive care unit is common, although how this decision is reached can be varied and arbitrary. Inevitably, the patient is unable to participate in this discussion because their capacity is limited by the nature of the illness and the effects of its treatment. Physicians often discuss these decisions with relatives in an attempt to respect the patient's wishes despite evidence suggesting that the relatives may not correctly reflect the patient's desires. Advance decisions, commonly known as 'living wills', have been proposed as a way of facilitating the maintenance of an individual's autonomy when they become incapacitated. Others have argued that legalising advance decisions is euthanasia by the back door. In October 2007 in England and Wales, advance decisions will become legally binding as part of the 2005 Mental Capacity Act. This has been the case in the USA for many years. The purpose of the present review is to examine the published literature regarding the effect of advance decisions in relation to the provision of adult critical care.