The stem cell is defined as that cell in a tissue which, under normal circumstances, maintains its own population, undiminished in function and size, and furnishes daughters to provide new functional cells of that tissue. The daughters may, or may not, have to undergo further differentiation and/or maturation in order to achieve their functional stage. The fundamental characteristic of a stem cell, therefore, is self-renewal. Evidence is presented which implicates the microenvironment as a major component of the stem cell system, without which stem cells cannot be maintained. Furthermore, it is suggested that stem cell properties do not reside in one specific cell type in the population but, when necessary, cells other than those normally playing the stem cell role, can have stem cell function imposed upon them by the appropriate microenvironment. The stem cell "niche" hypothesis is presented to explain the dependence of stem cells upon their microenvironment. The postulate is offered that there are no cells which are intrinsically stem cells but that a range of cells in a tissue possess stem cell potential to a greater or lesser extent.