Until recently, the identification of chromosome abnormalities that are responsible for congenital disorders and cancer has relied on the use of banded chromosome preparations and fluorescence in situ hybridization. The introduction of array-based comparative genomic hybridization (aCGH) is revolutionizing the field of cytogenetics. This University of Washington program highlights some of the recent discoveries made using aCGH, and addresses the advantages and disadvantages of this technology, particularly with regard to its use in the clinical cytogenetics laboratory. From the Series:Laboratory Medicine Grand Rounds