The new American Association on Mental Retardation (AAMR) definition of mental retardation (Luckasson et al., 1992) represents a radical departure from previous definitions. In the present paper we examined the extent to which the new definition provides decision rules to guide clinicians and researchers in classification efforts. We concluded that the IQ criterion of 75 will increase the proportion of the general population eligible near the cut-off score. Moreover, the 10 adaptive skill areas adopted fail to consider developmental factors and cannot be assessed reliably with current scales. Proposed differentiation by levels of needed supports is also challenged as being imprecise and not amenable to reliable measurement. Implications of this new definition for diagnostic practices and research endeavors were explored.