STEM cells are among the key players in regenerative medicine and stem cell therapy, and thus they are the keen focus of cell biologists’ interest. Unequivocal identification of specific stem cell types is the prerequisite for their purification by cell sorting and for follow-up of therapeutic regenerative strategies. The first stem cells to be identified were hematopoietic stem cells (HSCs). This was over four decades ago. Their key functional characteristics-self renewal and multilineage differentiation—have proved to be ubiquitous for all adult tissue— or organ-derived stem cells, and even for pluripotent embryonic, nuclear transfer, and induced pluripotent stem cells. Although the use of embryonic stem cells for research and therapeutic purposes raises various legal and ethical issues, and nuclear transfer and induced pluripotent stem cells are not yet characterized thoroughly enough to be used for such purposes, the adult human body appears to host in most of its tissues stem cells that can be extracted, propagated in vitro, and used for therapeutic purposes. In fact, these resident stem cells probably have as their biological role the maintenance and regeneration of the given tissue throughout the life of the individual. To fulfil this role, resident stem cells need to exhibit some specific properties in addition to the ability of self renewal, which is achieved by keeping at least one of the daughter cells as a stem cell during mitosis. Primarily, they need to be in an undifferentiated state, but possess high differentiation potential—implying the ability to differentiate into all cell types of their home tissue and possibly into other cell types as well, when appropriate (experimental) circumstances are provided. Furthermore, stem cells are a population of cells that are in a growth arrested state most of the time, but can re-enter the cell-cycle on demand-for example upon tissue injury-and give rise to a differentiating and highly proliferative progeny. These latter are termed progenitor cells or transient amplifying cells, and are very similar to stem cells (including some of their molecular markers) in most aspects, except they do not renew themselves and thus their population becomes terminally differentiated after a limited, though sometimes enormous number of cell divisions. In conjunction with slow cycling, stem cells require a stem cell niche: a microenvironment that provides external factors necessary for maintaining stem cell properties and functions, collectively often referred to as ‘‘stemness.’’ Although not a consensus yet, stem and progenitor cells are sometimes together termed precursors, indicating that both can be expanded under appropriate conditions. The discovery of some stem cells, like that of HSCs has come through observing the normal and pathological differentiation processes of ever-renewing tissues of the body and