The mammalian target of rapamycin (mTOR) senses nutrients and growth factors to coordinate cell growth, metabolism and autophagy. Extensive research has mapped the signaling pathways regulated by mTOR that are involved in human diseases, such as cancer, and in diabetes and ageing. Recently, however, new studies have demonstrated important roles for mTOR in promoting the differentiation of adult stem cells, driving the growth and proliferation of stem and progenitor cells, and dictating the differentiation program of multipotent stem cell populations. Here, we review these advances, providing an overview of mTOR signaling and its role in murine and human stem and progenitor cells.