In the past decade, we have witnessed a flood of reports about mutations that cause or contribute to intellectual disability (ID). This rapid progress has been driven in large part by the implementation of chromosomal microarray analysis and next-generation sequencing methods. The findings have revealed extensive genetic heterogeneity for ID, as well as examples of a common genetic etiology for ID and other neurobehavioral/psychiatric phenotypes. Clinical diagnostic application of these new findings is already well under way, despite incomplete understanding of non-Mendelian transmission patterns that are sometimes observed.