Differentiation of B lymphocytes involves the step-wise acquisition of a specialized phenotype that depends on the expression of lineage-specific genes and the repression of genes characteristic of multipotent progenitors and alternate lineages. The early steps of B lineage specification and commitment are, partly, controlled by the well-characterized transcription factors Ikaros, Pu.1, E2A, early B cell factor-1, and Pax5 that act in a complex regulatory network. However, our understanding of B cell differentiation is far from complete. Recent work has shed light on the mechanisms by which transcription factors implement cell type-specific gene expression patterns and epigenetic changes in chromatin that allow for B lineage specification and commitment.