Lymphocytes are essential for the generation of specific immunity. Development of B cells in the bone marrow and T cells in the thymus have several analogous features, and are tightly regulated processes. Even though there is an increasing amount of information concerning lymphopoiesis, a lot of questions remain. The aim of this thesis has been to understand some of the molecular events that contribute to the control of lymphocyte development. Expression of the B cell receptor is an important checkpoint in B lymphocyte development. The Dµ protein is a truncated B cell receptor that can induce some of the signals elicited by full length µ, but cannot promote further B cell differentiation. In order to determine if this could stem from an impaired survival signal, we introduced Bcl-2 into RAG2 deficient Dµ transgenic mice. Analysis of these mice showed that Dµ could not support pre-B cell maturation despite extended survival of B cell precursors by Bcl-2. In addition, data from recombination competent Dµ transgenic mice demonstrated that the Dµ induced partial block is permissive for marginal zone B cell development, whereas the formation of follicular B cells is severely reduced. The bHLH family of transcription factors is known to be involved in the regulation of lymphocyte development. Whereas the roles of E2A and HEB have been well documented in both B- and T-lymphocytes, detailed knowledge concerning E2-2 is lacking. To address the role of E2-2 in B cell development, we have reconstituted mice, using E2-2 deficient fetal liver cells, and analysed the B cell compartments. We also measured mRNA expression patterns for the three E-proteins in wildtype mice. Resulting data show that, in addition to a role in B cell lineage entry, E2-2 is required for efficient expansion of pro-B cells, and also influences the follicular versus marginal zone decision. While focusing on assigning a role for E2-2 in T-cell development, we analyzed the expression of the E-proteins during this process and performed functional studies in fetal thymic organ cultures. E2-2 deficient mouse embryos were shown to display a partial block at the DN3 stage, which was not due to proliferation or apoptosis defects. In addition, analysis of expression levels of the pre-Talpha chain suggests that E2-2 may play a role in the regulation of transcription of pre-Talpha, and therefore in the assembly of the pre-T cell receptor.