Infertility is both a private and a social health problem that can be observed in 12–15% of all sexually active couples. The male factor can be diagnosed in 50% of these cases either alone or in combination with a female component. The causes of male infertility can be identified as factors acting at pre-testicular, testicular or post-testicular level. However, despite advancements, predominantly in the genetics of fertility, etiological factors of male infertility cannot be identified in approximately 50% of the cases, classified as idiopathic infertility. On the other hand, the majority of the causes leading to male infertility can be treated or prevented. Thus a full understanding of these conditions is crucial in order to allow the clinical andrologist not simply to retrieve sperm for assisted reproductive techniques purposes, but also to optimize the male’s fertility potential in order to offer the couple the possibility of a spontaneous conceivement. This chapter offers the clinical andrologist a wide overview of pre-testicular, testicular, and post-testicular causes of male infertility.