DNA arrays can be powerful tools to investigate transcriptional regulation of biological systems in a highly parallel and genome-wide manner. Their use has furthered the investigation of basic cell and developmental biology and is now being applied toward the understanding and diagnosis of human disease. DNA arrays are of particular use in the study of circadian biology because the clock is, at its heart, a transcriptional/translational feedback loop. Here, we review the underlying technology behind DNA arrays, discuss general applications, and focus on the use of the arrays in the study of circadian biology in plants, flies, and mice.