Careful analysis of sperm morphology has always been an important part of a routine semen examination. However, the usefulness of sperm morphology assessment as a predictor of a man's fertilizing potential has often been challenged due to different classification systems, various slide preparation techniques and inconsistency of analyses within and between laboratories. Automated sperm morphology analysis instruments may overcome the subjective nature of visual assessments of sperm morphology, but the technical problems are numerous and the validity of these instruments has still to be proven. Having reviewed the literature, it seems clear that there is general agreement concerning the clinical relevance and predictive value of this single semen parameter in vivo and in vitro. Nevertheless, even in cases of severe teratozoospermia, fertilization may be possible. Studies on the acrosome reaction are very promising for patients with severe sperm morphology abnormalities that do not have major effects on the fertilizing potential. Most promising is the development of in-tracytoplasmic sperm injection (ICSI) as the treatment of first choice in cases of severe teratozoospermia with failed fertilization in vitro. Normal fertilization and pregnancy rates can be obtained with ICSI in the presence of extreme teratozoospermia, suggesting that sperm morphology may be important in spermato-zoa-zona binding, penetration and spermatozoon-oocyte fusion but fails to be of any predictive value once the spermatozoon reaches the cytoplasm of the oocyte. To conclude, accurate and strict sperm morphology assessment is very useful in evaluating a patient's fertilizing potential, with the main advantage that the methods used to examine this parameter are easy to learn on equipment found in most laboratories.