Array comparative genomic hybridization (array CGH) has revolutionized the cytogenetic testing available for patients with learning disabilities who have "chromosomal" phenotypes with dysmorphic features and multiple anomalies. Screening large patient cohorts with mental retardation by array CGH has recently lead to the characterization of many novel microdeletion and microduplication syndromes, initially according to the shared cytogenetic aberrations, with secondary characterization of the corresponding phenotypes. This review provides a detailed clinical and molecular cytogenetic description of several of the most common of these aberrations. We have chosen to focus on patients in whom the cytogenetic abnormalities were principally described by array CGH, rather than by G-banded karyotyping or fluorescence in-situ hybridization. The syndromes that we have chosen include the 17q21.31 deletion and 17q21.31 duplication syndromes, 15q13.3 deletion syndrome, 16p11.2 deletion syndrome, 15q24 deletion syndrome, 1q41q42 deletion syndrome, 2p15p16.1 deletion syndrome and 9q22.3 deletion syndrome. In time, we hypothesize that at least some of these will become as clinically well characterized and recognizable to the clinician as the commoner microdeletion syndromes today. Although the full extent of the phenotypes is still evolving for many of these novel microdeletions, it is clear that array CGH has heralded an unparalleled era of discovery for clinical cytogenetics.