This paper critically examines the best interests principle and its role in making decisions about intensive care treatment. In current practice the best interests principle is sometimes relied upon to guide decision making in circumstances when the patient is incompetent, although it is intrinsically linked to inconsistent assumptions about what is meant by quality of life. This situation means that there is potential that moral errors will be made that may result in an unwanted extension of life for some individuals or the premature death of others. It is difficult to justify such decision making on ethical grounds. A greater understanding of the best interests principle, and consequently the concept of quality of life, is needed in order to ensure that decision making about intensive care is ethically defensible. It is argued that an ideal theory of quality of life provides an appropriate framework for best interests decisions, and that the decision making process ought to, whenever possible, involve the patient's close family.