This paper considers the way in which the best interests test has been used in making health care decisions for mentally incompetent individuals in the Irish and English Courts where this is the test of choice. It looks in brief at the development of the test and the way in which the House of Lords has "de-theorised" the question of best interests in cases such as Re F (Mental Patient: Sterilisation) and Airedale N.H.S. Trust v. Bland, leaving little in the way of assistance for lower courts. The test adopted by the Irish courts has also lacked a strong theoretical underpinning. However, the existence of a written constitution has meant that there has at least been recognition of the role of rights in determinations such as In Re a Ward of Court. Having noted the inadequacies in the current test as it is applied, the article then considers the question of whether the best interests test has been inadequately applied or whether it is fatally flawed and should be dispensed with as a method of making determinations for mentally incompetent people. The article looks at two alternatives to the best interests test; namely, purely rights based decision making and substituted judgment. Both will be seen to be flawed and it will be argued that the best interests test should be maintained but that a more careful theoretical and analytical approach is required.