Over the past few decades genetic advances have resulted in increasing understanding of the etiology of intellectual disability (ID) syndromes while detailed phenotypic descriptions of behavior and mental disorders have become possible with validated and reliable assessment tools. Although the concept that certain ID syndromes are associated with a recognizable physical, behavioral, and cognitive “phenotype” is not new, it has gained considerable popularity among psychiatrists and psychologists working with this population. In this chapter, we will review the key aspects of behavioral phenotypes and genetic testing before describing some of the phenotypes associated with specific syndromes. For terms in italics, see “Glossary.”