Male fertility-the ability to produce viable sperm that are able to support fertilization and oocyte activation and to sustain development during embryogenesis and beyond-is essential for success of mammalian reproduction and development. Production of quality semen depends on the existence of an effective male reproduction system that is consistently able to produce viable gametes. This requires a male with a fertility phenotype that has evolved under the influence of its genetics, environment, and epistasis to a high degree of efficacy. As the sperm from one bull can be used to inseminate tens of thousands of cows, bull fertility has an enormous influence on the efficiency and success of the cattle breeding and reproduction. Even among bulls that produce ample amounts of sperm having apparent normal morphology, some of these animals will still not exhibit high fertility. Despite large investments to study the causes of low fertility as well as research on methods to prevent, diagnose, and treat subpar male fertility, only limited success has been achieved, and the problem remains poorly defined. The objectives of this paper are to review the male reproductive system and the endocrine regulation of male fertility. It will explore the functions of specific hormones regulating male fertility, hormonal control of sperm viability, as well as environmental factors influencing male fertility. The bovine will be the species of focus, but key aspects of male fertility of other mammals (pigs, goats, horses, monkeys and humans) will be included. This review is intended to be a useful source of information for both basic and applied research, as well as for animal production and human andrologists. © 2013 by Nova Science Publishers, Inc. All rights reserved.