Array-based comparative genomic hybridization (array CGH), a method used to detect gains or losses of genetic material, has recently been applied to prenatal diagnosis of genomic imbalance in the clinical laboratory setting. This new and exciting diagnostic tool represents a major technological step forward in cytogenetic testing and addresses many of the limitations of current cytogenetic methods. Conventional chromosome analysis, the current gold standard in prenatal diagnosis, focuses primarily on the detection of common aneuploidies and is limited by its capacity to detect only those copy number changes that are large enough to be microscopically visible (typically 5-6 Mb in size at the 500 band level). In contrast, array CGH analysis simultaneously evaluates regions across the entire genome and allows for detection of unbalanced structural and numerical chromosome abnormalities of less than one hundred kb. Array CGH analysis also overcomes some of the limitations of chromosome analysis, such as the requirement for cell culture and longer reporting time, by using direct uncultured fetal specimens. With many diagnostic laboratories now embracing this technology, the past year has seen tremendous growth in the use of array CGH analysis for prenatal diagnosis. This review aims to summarize array CGH methodology and its current applications in prenatal diagnosis.