Microchimerism has been defined by the presence of a low number of circulating cells transferred from one individual to another. The transfer of microchimeric cells naturally takes place during pregnancy and occurs bi-directionally between the mother and fetus. Further, microchimerism can also be a result of blood transfusions and organ transplants. Microchimeric cells have been implicated in health and disease. Fetal microchimerism has been correlated with the hyporesponsiveness of the maternal immune system towards a fetal allograft and with the longevity of organ transplants. However, microchimeric cells have been implicated in the pathogenesis of autoimmune diseases including systemic sclerosis. In contrast, microchimeric cells were found to contribute to tissue repair. Much controversy exists around the role of microchimeric cells in the pathogenesis of certain diseases, and these cells in tissues may be a consequence rather than the cause of disease.