The current dogma is that the thymus is colonized by progenitors that retain the capacity to generate both T cells and B cells, and that intrathymic Notch signalling determines lineage choice so that T cells, rather than B cells, develop in the thymus. However, evidence is now accumulating to indicate that, at least during fetal life, this is not the case. Rather, it now seems that the fetal thymus is colonized by progenitors that have already made the T-cell versus B-cell lineage choice. We propose an alternative role for Notch signalling in the thymus, which is not to mediate this choice but instead to reveal it by supporting further T-cell differentiation in the thymic microenvironment.