It is helpful for the new student of autism to know the clinical and research basis from which our definition of the syndrome has been evolved. The main features and characteristics are no less meaningful for being recognized as working definitions, subject to alteration with the discovery of new knowledge about causation and treatment. Such research can be most useful, even when samples of autistic children with different specific characteristics are selected. However, it is necessary for the specific characteristics, and their selection purpose, to be identified explicitly. Likewise, the limits of applying the findings on such subsamples to other autistic children should be frankly acknowledged.