The neurodevelopmental disorder Rett syndrome was first described in 1966 by Andreas Rett, who described girls with loss of speech and hand use displaying characteristic hand stereotypies. Since then, the disease has been linked to mutations in the gene MECP2. However, the basis of the diagnosis is still clinical as defined by the latest clinical criteria as proposed by Neul and colleagues in 2010. This article presents a short clinical and molecular overview of the latest in Rett syndrome with emphasis on the Danish patients, headlines for making the diagnosis, differential diagnoses and molecular diagnostic possibilities.