This chapter describes methods that allow researchers to localize sites of gene expression at both the mRNA and protein levels, within a histological section of a single tissue or tissue microarrays. Identification of the cells within a tumor specimen that express a specific mRNA (assessed by hybridization in situ ) or protein (assessed by immunohistochemistry) is a significant step toward understanding tumor behavior that is not possible using methods in which tissues are homogenized before analysis. Combined detection of mRNA and protein may permit effects of subtle regulatory processes such as translational repression to be observed and allow coexpression of genes to be detected. Application of such combined techniques to tissue microarrays of tumor tissues from cohorts of patients with known clinical outcome would allow the predictive value of specific patterns of expression to be tested retrospectively.