Microarray technology is revolutionizing the assessment of gene expression in human disease. Coupled with the publication of the human genome, it is becoming possible to measure the relevant levels of all 30,000-40,000 human genes in a single experiment. The applications of these methods to breast cancer research are helping us to both further our understanding of the biology of mammary tumorigenesis, as well as providing novel subclassifications of breast cancer of direct clinical relevance. Unbiased RNA amplification techniques are allow the study of small biopsy samples as well as archival pathology specimens, increasing our molecular understanding, and providing real hope for the clinical application of gene expression profiling.