&NA;Chromosome deletion and microdeletion syndromes account for an increasing number of clinically recognizable genetic conditions. New deletion syndromes continue to be characterized, and a number of previously described syndromes are being found to be due to chromosomal deletions or microdeletions. Fluorescentin situhybridization technologies are in wide clinical use to diagnose deletion and microdeletion syndromes, and future uses of these technologies will provide prognostic information for patients and their parents, as the genes responsible for the phenotypic aspects of various deletion syndromes are identified. Future research studies will focus on delineating critical deletion intervals at a molecular level, and identifying candidate genes for the phenotypic features of deletion and microdeletion syndromes, toward the goal of understanding the pathology of the abnormal developmental and physiologic processes involved in each syndrome.