The preceding chapters by Wolf and Paterson and Klein-Tasman and Janke reveal similarities as well as differences between the related fields of intellectual disabilities and autistic spectrum disorders, and suggest distinct as well as overlapping areas for continued study. Both sets of authors note difficulties inherent in the fact that the disorders are defined by behavioral characteristics that have been subject to change over time. In the field of intellectual disability, confusion has been most marked in the debate over the statistical cut-off for the ID diagnosis, and over the role of adaptive skills in the conceptualization of the disability. In 1992, the American Association for Mental Retardation recommended changing the IQ cut-off from 70 to 75, a decision that would have doubled the number of individuals diagnosed with ID [1]. After a period of considerable debate, clinicians and researchers retained the cut-off score of 70 for both cognitive measures and adaptive skills. Klein-Tasman and Janke describe several caveats to this decision, including recognition of the psychometric limitations of IQ tests, and the danger of excessive reliance on overall scores and attendant disregard for evidence of variability in cognitive function. Similarly, despite dissenting opinions, deficits in adaptive functioning are now regarded as central to the diagnosis. Klein-Tasman and Janke note that this decision, while useful, introduces a variety of concerns, including questions about the validity of adaptive measures and the extent to which they reflect variance unique to adaptive functioning and distinct from cognitive ability.