It is estimated that 12%–15% of sexually active couples are infertile. When broken down by gender, a male component can be identified approximately 50% of the time either in isolation or in combination with a female factor. Previous research in a US male fertility clinic analyzing 1430 patients identified causes of infertility from most to least common: varicocele, idiopathic, obstruction, female factor, cryptorchidism, immunologic, ejaculatory dysfunction, testicular failure, drug effects/radiation, endocrinology, and all others. However, despite recent technologic and diagnostic advances, idiopathic infertility remains a common diagnosis, with approximately 25% of patients not having an identifiable cause of infertility. Regardless, many recognizable causes of male infertility are treatable or preventable; thus, a keen understanding of these conditions is paramount. This chapter comprises an overview of etiologies of male infertility, divided into pre-testicular, testicular, and post-testicular causes.