Toxicogenomics has been described as a discipline combining expertise in toxicology, genetics, molecular biology, and environmental health to elucidate the response of living organisms to stressful environments. It includes the study of how genomes respond to toxicant exposures and how genotype affects responses to toxicant exposures. As the technologies for monitoring these responses rapidly develop, it is critical that scientists and regulators are confident that the technologies are reliable and reproducible and that the data analyses have been validated. To discuss these issues in a public forum, the Committee on the Validation of Toxicogenomic Technologies designed a workshop to consider the current practice and advances in the validation of toxicogenomic technologies. The workshop focused on the technical aspects of validation, recognizing it as a prerequisite for considering other important issues, such as biological validation (e.g., validating the use of microarray "signatures" to describe a toxic effect).