Gene expression is critical for tumor initiation and progression. However, we lack a comprehensive understanding of all genes that are aberrantly expressed in human cancer. Recently, microarrays have been used to obtain global views of human cancer gene expression and identify genetic markers that might be important for diagnosis and therapy. The initial sequencing of the human genome, coupled with technological advances, now make it possible to embrace the genetic complexity of common human cancers in global fashion. Tools are currently available, or are being developed, for the identification of all changes that take place in cancer at the DNA, RNA, and protein levels. In particular, the use of microarrays for the comprehensive analysis of RNA expression in human tumor samples holds much promise.