A DNA microarray is a solid support such as a glass slide, silicon chip or nylon membrane on which DNA molecules are attached at precise locations. Using DNA microarrays, the expression of tens of thousands of genes in a biological sample can be detected in one experiment. Emerging data suggests that the use of DNA microarrays can aid the differentiation of tumors with similar morphological appearance, predict patient outcome independently of conventional prognostic factors and select for response or resistance to specific anti-cancer therapies. DNA microarray technology thus has the potential to supplement standard diagnostic procedures in oncology and permit a more individualized approach to patient management. Prior to clinical application, however, this methodology must be simplified, standardized, evaluated in external quality assessment schemes and made available at relatively low costs. Most importantly, the preliminary, but promising, early findings must be validated by high-level evidence studies, such as large prospective randomized trials or meta-analyses.