This review by Geoffrey Brown (University of Birmingham) revisits the long-standing question of when hematopoietic stem cells (HSCs) become lineage-affiliated during hematopoiesis. The classical model posits that lineage commitment occurs late and proceeds stepwise, through a hierarchy of oligopotent progenitors that progressively restrict their differentiation potential until unipotent progenitors are formed. Brown contrasts this with an alternative model in which lineage affiliation is established much earlier, potentially within the HSC compartment itself, after which development proceeds as a continuous, rather than stepwise, process. The article evaluates the evidence supporting each model and discusses the extent to which lineage-affiliated HSC subsets self-renew and contribute substantially to steady-state hematopoiesis, with implications for how hematopoietic hierarchies are conceptualized