Genomic-scale experimentation aims to view biological processes as a whole, yet with molecular precision. Using massively parallel DNA microarray technology, the mRNA expression of tens of thousands of genes can be measured simultaneously. Mathematical distillation of this flood of gene expression data reveals a deep molecular and biological logic underlying gene expression programs during cellular differentiation and activation. Genes that encode components of the same multi-subunit protein complex are often coordinately regulated. Coordinate regulation is also observed among genes whose products function in a common differentiation program or in the same physiological response pathway. Recent application of gene expression profiling to the immune system has shown that lymphocyte differentiation and activation are accompanied by changes of hundreds of genes in parallel. The databases of gene expression emerging from these studies of normal immune responses will be used to interpret the pathological changes in gene expression that accompany autoimmunity, immune deficiencies, and cancers of immune cells.