The processes of abnormal sperm penetration and incorporation into human oocytes during IVF and after sperm microinjection, assessed by TEM, are reviewed. A spectrum of morphologically abnormal sperm with head, neck and midpiece defects penetrate the egg vestments of oocytes (1-3 h after insemination with sperm from normal donors) in both unfertilized and normally fertilized oocytes. Sperm with aberrant head shapes, acrosomal and nuclear defects penetrate the outer zona pellucida but are rarely encountered in the inner zona and perivitelline space, showing that the zona prevents abnormal sperm penetration. Grossly abnormal sperm are, however, incorporated into the ooplasm of zona-denuded oocytes. Microinjection of poor-quality sperm from male-factor patients into the perivitelline space or directly into the ooplasm of oocytes also reveals a variety of structural defects conforming to those observed in washed sperm pellets, highlighting the difficulties of selection of 'normal' sperm for microinjection. Abnormal sperm have been seen to interact and fuse with the oocyte in the perivitelline space, and to be incorporated into the ooplasm. Those with nuclear and neck (centriolar) defects are of particular significance, as they might contribute to aberrant development. The functional competence of immotile, round-headed and epididymal sperm is also briefly discussed.