The thymus produces new T cells throughout life but has no self-renewing ability and requires replenishment and recruitment of progenitors derived from the bone marrow. Despite the progress in delineation of mature blood cell development several questions remain regarding T lymphopoiesis. Understanding the developmental stages from multipotent hematopoietic stem cells (HSCs) to the T-cell lineage-restricted progenitors has many potential clinical implications as it is important for understanding malignant transformation in T-cell cancer, accelerating T-cell regeneration after bone marrow transplantation and chemotherapy, and establishing new therapies to treat T-cell immune deficiencies. This review focuses on the steps leading from the HSCs in the bone marrow to the lineage committed T cells inside the thymus.