Mental retardation is a lifelong human disability characterized by impairment of cognitive and adaptive skills. It is also one of the main causes of handicap among children and young adults. Relatively high prevalence of mental retardation, extensive investigations that still haven''t been able to give an accurate genetic diagnosis to majority of the families, high cost and burden to the society and families – this renders mental retardation one of the most important unsolved problems in medicine. This book describes two of the common X-linked mental retardation syndromes – fragile X syndrome and creatine transporter defect with prevalence and clinical problems. Furthermore, association with clinical presentation has been shown in two rare small submicroscopic aberrations. Up-to-date scientific information about mental retardation, fragile X syndrome, creatine transporter defect with the information about methodologies that are being utilized to diagnose different genetic diseases can be found. This book should be especially useful to scientists and clinicians, or anyone else who has interest in developmental problems in children and adults.