A novel family of transcription factors that appears to play a critical role in the development of all animal species was recently uncovered on the basis of homology to the DNA binding domain of the Brachyury, or T locus, gene product. Phylogenetic studies have shown the ancient origin of this gene family, which has been named the T-box family, prior to the divergence of metazoa from a common ancestral type. T-box genes have now been identified in the genomes of C. elegans, Drosophila, sea urchin, ascidian, amphioxus, Xenopus, chick, zebrafish, mouse, and human and will probably be found in the genomes of all animals. Although functional analyses of T-box family members have just begun, the results show a wide range of roles in developmental processes that extend over time from the unfertilized egg through organogenesis. Only a few mutations in T-box genes are known, but all have drastic effects on development, including a targeted mutation in mice causing an embryonic lethal phenotype, and two human T-box gene mutations that results in developmental syndromes. This review presents a current overview of progress made in the analysis of T-box genes and their products in a variety of model systems.