Asperger syndrome is the term applied to the mildest and highest functioning end of what is known as the spectrum of pervasive developmental disorders (or the Autism spectrum). Like other conditions along that spectrum it is felt to represent a neurologically-based disorder of development, most often of unknown cause, in which there are deviations or abnormalities in three broad aspects of development: social relatedness and social skills, the use of language for communicative purposes, and certain behavioral and stylistic characteristics involving repetitive or perseverative features and a limited but intense range of interests. It is the presence of these three categories of dysfunction, which can range from relatively mild to severe, which clinically defines all of the pervasive developmental disorders, from AS through to classic Autism. Although the idea of a continuum of PDD along a single dimension is helpful for understanding the clinical similarities of conditions along the spectrum, it is not at all clear that Asperger syndrome is just a milder form of Autism or that the conditions are linked by anything more than their broad clinical similarities. Asperger syndrome represents that portion of the PDD continuum which is characterized by higher cognitive abilities (at least normal IQ by definition, and sometimes ranging up into the very superior range) and by more normal language function compared to other disorders along the spectrum. In fact, the presence of normal basic language skills is now felt to be one of the criteria for the diagnosis of AS, although there are nearly always more subtle difficulties with pragmatic/social language. Many researchers feel it is these two areas of relative strength that distinguish AS from other forms of Autism and PDD and account for the better prognosis in AS. Developmentalists have not reached consensus as to whether there is any difference between AS and what is termed High Functioning Autism (HFA). Some researchers have suggested that the basic neuropsychological deficit is different for the two conditions, but others have been unconvinced that any meaningful distinction can be made between them. One researcher, Uta Frith, has characterized children with AS as having "a dash of Autism." In fact, it is likely that there may be multiple underlying subtypes and mechanisms behind the broad clinical picture of AS. This leaves room for some confusion regarding diagnostic terms, and it is likely that quite similar children across the country have been diagnosed with AS, HFA, or PDD, depending upon by whom or where they are evaluated.

Since AS itself shows a range or spectrum of symptom severity, many less impaired children who might meet criteria for that diagnosis receive no diagnosis at all and are viewed as "unusual" or "just different," or are misdiagnosed with conditions such as Attention Deficit Disorder, emotional disturbance, etc. Many in the field believe that there is no clear boundary separating AS from children who are "normal but different." The inclusion of AS as a separate category in the new DSM-4, with fairly clear criteria for diagnosis, should promote greater consistency of labeling in the future.