Cellular therapy has become a billion-dollar industry and is set to become one of the therapeutic pillars of healthcare in the 21st century. Adult stem cells, which include haematopoietic stem and progenitor cells (HSPCs) and mesenchymal stromal/stem cells (MSCs), is one of the major cell types currently under investigation for use in cell therapy. This review focuses on HSPCs and MSCs and discusses their heterogeneous nature and the problems faced in expanding these cells to therapeutic numbers for use in clinical applications.