Lymphocyte antigen receptor genes are assembled through the cutting and joining of segments of DNA in developing lymphocytes. The basic features of the biochemical steps of this assembly process, referred to as the V(D)J recombination, are similar for the assembly of all lymphocyte antigen receptor genes, yet this assembly is precisely regulated in several important contexts during lymphocyte development. It has long been appreciated that this occurs through modulation of accessibility of antigen receptor loci to the enzymatic complex that assembles antigen receptor genes. However, recent studies have suggested that some regulatory constraints may be enforced at the level of the V(D)J recombination reaction itself. This review focuses on recent advances in the understanding of the regulation of antigen receptor gene assembly, with particular attention paid to the assembly of T-cell receptor β-chain genes during T-cell development. © 2005 Humana Press Inc.