Rett Syndrome is one of the least commonly occurring autism spectrum disorders (ASD), but certainly one of the most devastating. A genetic profile has been identified, but checklists still have an important role for prescreening, especially before expensive genetic testing, and to provide precise strengths and weaknesses with respect to the core features of the disorder. Furthermore, research is now demonstrating subprofiles of genetic mutation which may be linked to profiles of behavioral responding and general symptom profiles. We review the literature on the nosology and assessment of Rett Syndrome in light of these developments. Specific symptoms and assessment techniques are discussed and potential future research avenues are reviewed with an eye to strengths and weaknesses of the current knowledge base.