In the last decade, advances in the field of genetics have contributed greatly to our understanding of intellectual disability. The human genome mapping project and gene knockout animal studies have enabled scientists to map specific intracellular changes associated with gene mutations associated with cognitive deficits. Improved diagnostic methods, including high-resolution karyotyping, subtelomeric screening, chromosome microdissection, nuclear magnetic resonance spectroscopy (MRS), and fluorescence in situ hybridization (FISH) have made it possible to determine etiology in many more cases of intellectual disability, including milder forms of the disorder (Vasconcelos 2004). Further, specific genes have been identified, such as MECP2, responsible for over 80% of cases of Rett syndrome (Amir et al. 1999). Below, we review some common genetic syndromes associated with intellectual disability.