Microarray experiments produce a large volume of data that require validation. The validation of these experiments can be carried out in many fashions. In the reduction to clinical utility, the use of tissue microarrays has become a common tool to both validate and generalize the results of microarray experiments. A tissue microarray is a collection of tissue specimens presented on a glass slide in a grid layout. Tissue microarrays contain between tens and hundreds of samples allowing the generalization of microarray findings to a large number of samples. Tissue microarrays can be used for in situ hybridization and immunohistochemical analysis, confirming the results of microarray experiments at both the transcriptional and the proteomic level. This chapter reviews the theory and application of tissue microarrays in the validation of microarray experiments.