The special property of stem cells is that their development is asymmetric. They give rise both to cells that are identical to themselves and to cells that are different. The mechanism that provides this asymmetry may be intrinsic or extrinsic. Such mechanisms are considered within the context of other systems where asymmetric development occurs. The specification of mating types in yeast provides a clear example of a stem cell system generated intrinsically. In fission yeast it appears that the asymmetry is due to chromosomal differences: this is the only known mechanism for intrinsic asymmetry. While there is good evidence for intrinsic asymmetry in both plants and invertebrates--particularly the nematode--the mechanism is not known. In insects and vertebrates there is no well established example of intrinsic asymmetry if one excludes asymmetric cytoplasmic localization during cleavage of the egg. Asymmetry is thus due to environmental influences. Stem cell systems are usually well structured and the cell's behaviour seems to be position-dependent. This is well established for the stem cells of hydra. By contrast it is claimed that the mammalian haemopoietic system is generated by an intrinsic, asymmetric, probabilistic mechanism--the validity of this view is questioned.