The B cell development from hematopoietic stem cells is a continuous and highly regulated process where multiple differentiation potentials are gradually lost while acquiring lineage specialized functions. At 50 years of the B cell discovery, the current knowledge of its early differentiation largely derive from the isolation and characterization of bone marrow early progenitor cells initiating the lymphoid program, and from the definition of transcriptional activity patterns that control cell fate decisions. Of particular relevance has been the intercommunication between B cell precursors and key components of the hematopoietic microenvironment, both for generation of novel models integrating all regulatory elements of this complex process, and for the understanding of this branch of the adaptive immune system in disease settings. This review provides an overview of the complex process of lymphoid differentiation: from the hierarchical organization and biological characteristics of primitive cells involved in its earliest stages, to the principles governing its interdependence with the hematopoietic microenvironment.