Recent studies indicate cells transfer between fetus and mother during pregnancy and can persist in both decades later. The presence within one individual of a small population of cells from another genetically distinct individual is referred to as microchimerism. Naturally acquired microchimerism has recently been investigated in autoimmune diseases, including scleroderma, thyroiditis, primary biliary cirrhosis, Sjögren syndrome, systemic lupus, dermatomyositis, and neonatal lupus. Iatrogenic chimerism has been investigated in transplantation and following blood transfusion. Considering findings of naturally acquired microchimerism along with iatrogenic microchimerism suggests microchimerism can have detrimental and/or beneficial effects in both settings. Recent identification of tissue-specific microchimerism either from naturally acquired or iatrogenic microchimerism (eg, cardiac myocytes) raises the possibility that microchimerism can be a target of autoimmunity or alternatively contribute to tissue repair. Advances in this new frontier of research with varied and numerous implications for human health are summarized.