Worldwide, at least 45 million couples are suffering from infertility. In about 50 % of couples, a male factor is involved mainly because of errors that have occurred during male gametogenesis. Spermatogenesis is a complex process that already starts in early fetal life and continues during a man’s entire lifespan. Spermatogenesis involves cell specification, cell migration, mitotic and meiotic cell divisions, differentiation and eventually maturation. Only when all these events take place in an exact sequence, in a specific setting and without any errors, enough mature haploid spermatozoa will be produced enabling both fertilization of an oocyte and embryonic development. In this chapter, we will summarize the main events occurring during normal spermatogenesis along with potential errors that might arise at specific stages. In addition, the consequential fertility problem(s) will be described, together with their possible treatments either clinically available or under investigation.