DNA microarrays represent an important new tool to analyze human tissues. The technology enables investigators to measure the expression of several thousand mRNA species simultaneously in a biological specimen. This process, called transcriptional profiling, represents a technological breakthrough in the analysis of biological specimens. It may be used to screen for individual genes that are differentially expressed between normal and diseased tissues in the hope of finding novel targets for drug development or finding new single-gene markers of clinical outcome. Microarrays are also applied to learn about the complex biology of cancer by simultaneously monitoring interactions between hundreds of genes during experimental conditions in vitro or during therapy in vivo. Analysis of gene expression patterns may also be used as a classification tool to sort cancer into various clinically relevant subgroups that is not currently possible with other methods. The first clinically important applications of this technology will likely be its use as a tool to refine diagnosis and improve the accuracy of predictions of prognosis and response to therapy. DNA microarrays in several "proof-of-principle" experiments have demonstrated that they can predict important clinical outcomes, including outcomes that cannot currently be predicted with other methods, but the true clinical utility and the limits of this exciting new technology are yet to be established. This paper reviews the current methodology and applications of this technique as they relate to clinical oncology.