Knowledge of the overall gene expression profile within a cell can provide key insights into cellular physiology. Therefore, it is not surprising that a great deal of effort has been devoted to measuring the expression level of all of the genes in a cell. With the advent of DNA microarray technology, researchers now have the ability to collect expression data on every gene in a cell simultaneously. The vast datasets created with this technology are providing valuable information that can be used to accurately diagnose, prevent, or cure a wide range of genetic and infectious diseases. In this article, we describe the key technologies for measuring gene expression and discuss how these approaches can be used to identify the correlation between the observed expression patterns and disease. Additionally, we will highlight the key ethical considerations and discuss how these tremendously powerful technologies may impact society. (c) 2004 Elsevier Ltd. All rights reserved.