Helix-loop-helix (HLH) proteins are transcriptional regulators that control a wide variety of developmental pathways in both invertebrate and vertebrate organisms. Results obtained in the past decade have shown that HLH proteins also contribute to the development of lymphoid lineages. A subset of HLH proteins, the 'E proteins', seems to be particularly important for proper lymphoid development. Members of the E protein family include E12, E47, E2-2 and HEB. The E proteins contribute to B lineage- and T lineage-specific gene expression programs, regulate lymphocyte survival and cellular proliferation, activate the rearrangement of antigen receptor genes and control progression through critical developmental checkpoints. This review discusses HLH proteins in lymphocyte development and homeostasis.