Haematopoietic stem cells (HSCs), a very rare cell type in the bone marrow, are responsible for the life-long production of all cells of the blood including T and B cells. Until recently, it was thought that the differentiation of HSCs into the various haematopoietic cells was rather hierarchical in that differentiation along a given lineage was associated with a progressive loss of potential to give rise to other blood cell lineages. The recent development of very sensitive and quantitative in vitro assays, together with the identification of new progenitor subpopulations, has challenged this idea. Thus, lymphocyte progenitors can be shown to keep their developmental potential to give rise to myeloid, dendritic and NK cells until just prior to their final commitment stage. Here we review these new findings and concepts.