Infertility affects 13-18% of couples and growing evidence from clinical and epidemiological studies suggests an increasing incidence of male reproductive problems. The pathogenesis of male infertility can be reflected by defective spermatogenesis due to pituitary disorders, testicular cancer, germ cell aplasia, varicocele and environmental factors or to defective sperm transport due to congenital abnormalities or immunological and neurogenic factors. Recent studies suggest an increased incidence of genetic disorders related to male infertility which may affect different levels, interfering with germ cell generation and maturation or leading to the production of non-functional spermatozoa. The identification of genetic causes of male infertility raises the issue of the transmission of defects to the offspring, a situation that is becoming more important given the increasing use of intracytoplasmic sperm injection (ICSI), a procedure in which the natural selection of the spermatozoa is by-passed. Fertilization can occur in vitro using ejaculated, epididymal or testicular spermatozoa, either fresh or frozen-thawed, providing opportunities hitherto not possible for men to be genetic fathers.