Early onset intellectual disability (ID) is one of the largest unsolved problems of health care. Yet, it has received very little public attention in the past because many health care professionals do not perceive it as a health condition but as a social or educational issue. In severe ID, cytogenetically visible chromosomal abnormalities like trisomy 21 continue to be common, but since the introduction of array CGH, it is becoming clear that submicroscopic deletions and duplications are equally frequent, yet previously overlooked causes of ID. Until recently, the search for gene defects causing ID has focused on the X-chromosome. So far, >80 genes have been implicated in X-linked ID, largely owing to coordinated efforts of international consortia, and mutations in these genes account for >50% of the families with this condition. Autosomal forms, either due to dominant de novo mutations or to recessive gene defects, are presumably (far) more common than X-linked ones, and their molecular elucidation is a new challenge for research in this field. As recently shown, autosomal recessive ID (ARID) is extremely heterogeneous, and common forms are unlikely to exist. Ongoing studies into the function of ID genes are shedding more light on the pathogenesis of this disorder, and there is reason to believe that at least some genetic forms of ID may be amenable to drug treatment.