This paper distinguishes four major types of futility (physiological, imminent demise, lethal condition, and qualitative) that have been advocated in the literature either in a patient dependent or a patient independent fashion. It proposes five criteria (precision, prospective, social acceptability, significant number, and non-agreement) that any definition of futility must satisfy if it is to serve as the basis for unilaterally limiting futile care. It then argues that none of the definitions that have been advocated meet the criteria, primarily because their proponents have not paid sufficient attention to the problematic nature of the data supporting the use of their definitions.