During pregnancy there is transplacental traffic of fetal cells into the maternal circulation. Remarkably, cells of fetal origin can then persist for decades in the mother and are detectable in the circulation and in a wide range of tissues. Maternal CD8 T cell responses directed against fetal antigens can also be detected following pregnancy. However, the impact that the persistence of allogenic cells of fetal origin and the maternal immune response towards them has on the mother's health remains unclear and is the subject of considerable investigation. The potentially harmful effects of fetal microchimerism include an association with autoimmune disease and recurrent miscarriage. Beneficial effects that have been explored include the contribution of persistent fetal cells to maternal tissue repair. A link between fetal microchimerism and cancer has also been proposed, with some results supporting a protective role and others, conversely, suggesting a role in tumour development. The phenomenon of fetal microchimerism thus provokes many questions and promises to offer further insights not only into the biology of pregnancy but fields such as autoimmunity, transplantation biology and oncology.