THE FASCINATION The male germ cell is the only cell of the human organism that leaves the body when it has achieved its final, highly sophisticated structure and properties. The male germ cell is designed for one purpose only: to reach the female gametes and to fertilize them. The various stages in the development of the male germ cell are characterized by proliferative phases, by the recombination of the maternal and paternal chromosomes, and by the differentiation and development of a specialized transport vehicle, the spermato­ zoon. Furthermore, the establishment of a special pool of stem cells, the spermatogonia, guarantees the continuity of the sperm-generation process from puberty to old age. THE FATE OF THE MALE GERM CELL The destiny of any individual germ cell is determined by a program that we know only in fragments. On the one hand every human male is able to produce many billions of germ cells in his lifetime, yet the chance of any single sperm reaching and fertilizing the female germ cell is exceedingly rare. A fertility disturbance means that somewhere during the complicated playing out of the germ cell program mistakes are made, and the program fails. It is still a fact that more than 50% of men presenting with male factor infertility have to be diagnosed as idiopathic, largely because of our lack of knowledge and conse­ quent lack of appropriate diagnostic tools.