Malignant transformation results from accumulation of genetic and epigenetic events. Functional studies of cancer will be crucial to our understanding of its complexity and polymorphism. There is no doubt that emerging genomic and proteomic technologies will facilitate such investigations. Microarray technology is a new and efficient approach to extract data of biomedical relevance for a wide range of applications. In cancer research, it will provide high-throughput and valuable insights into differences in an individual's tumor as compared with constitutional DNA, mRNA expression, and protein expression and activity. Across individuals, comparisons could provide tissue-specific disease signatures that provide diagnosis based on hundreds of informative genes. The resulting product should be a wealth of tumor-associated and tumor-specific biomarkers, which may help in cancer etiology, diagnosis, and therapy and ultimately lead to "molecular nosology" of cancers. This review highlights the recent developments in microarray technologies in cancer research, focuses on the results obtained so far, and describes the eventual use of microarray technology for clinical applications.