We have used gene targeted mutational approaches to assess the role of the T cell receptor α (TCRα) enhancer (Eα) in the control of TCRα and TCRδ gene rearrangement and expression. We show that Eα functions in cis to promote Vα to Jα rearrangement across the entire Jα locus, a distance of greater than 70 kb. We also show that Eα is required for normal αβ T cell development; in this lineage, Eα is required for germline Jα expression, for normal expression levels of rearranged VαJα genes, and for expression of a diverse Vα repertoire. In γδ T cells, Eα is not required for VδDJδ rearrangement, but, surprisingly, is required for normal expression levels of mature VδDJδ transcripts and for expression of germline Jα transcripts. Our findings imply that Eα function is not limited to the TCRα components of the TCRα/δ locus or to the αβ lineage; rather, Eα function is important in both αβ and γδ lineage T cells.