IVF has become a well-established treatment for certain types of infertility, including long-standing infertility due to tubal disease, endometriosis, unexplained infertility or infertility involving a male factor. Soon it became clear that certain couples with severe male-factor infertility could not be helped by conventional IVF. Extremely low sperm counts, impaired motility and/or abnormal morphology represent the main causes of failed fertilization in conventional IVE In order to tackle this problem, several procedures of assisted fertilization based on micromanipulation of oocytes and spermatozoa have been established. The evolution of these techniques started with partial zona dissection (PZD), followed by subzonal insemination (SUZI) and finally led to the procedure of intracytoplasmic sperm injection (ICSI). ICSI represents the injection of a single spermatozoon directly into the ooplasm, thereby crossing not only the zona pellucida but also the oolemma. In 1992, the first human pregnancies and births after replacement of embryos generated by this novel procedure of assisted fertilization were reported (Palermo et al., 1992). The use of PZD had become controversial and was subsequently abandoned by many workers. Fertilization rates after ICSI were reported to be significantly better than after SUZI (Van Steirteghem et al., 1993a). Moreover, ICSI resulted in the production of more embryos with higher implantation rates, in comparison with SUZI (Van Steirteghem et al., 1993b). As a result, ICSI has been used worldwide and successfully to treat infertility due to impaired testicular function or obstruction of the excretory ducts resulting in severe oligoasthenoteratozoospermia or even azoospermia in the ejaculate. Successful IVF depends on the presence in the ejaculate of a certain number of spermatozoa with good motility and morphology. Riedel et al. (1989) reported minimum andrological requirements for in-vitro fertility by means of conventional IVF: 5 x 106/ml total count, 30% progressive motility and 30% normal morphology. Men with sperm parmeters below these values were considered to have a poor prognosis. Today, however, the most efficient procedure to treat this type of male infertility is ICSI: only one motile (live) spermatozoon is required per mature metaphase II oocyte to be injected. A