The reconstitution of T-cell populations is a critical component of immune recovery after allogeneic stem cell transplantation. Recent studies have used new techniques to focus on the interplay of thymopoiesis and peripheral expansion that defines T-cell repopulation. Peripheral expansion, driven by host cytokines and antigenic stimulation, dominates early recovery. However, this expansion is often transient and is characterized by limited repertoire diversity. Renewed thymopoiesis has been found to play a critical role in the recovery of repertoire diversity and stable repopulation. The insights gained into the regulation of these processes may provide new therapies to enhance recovery.