From the last decade, the physiological role of relaxin in male has been made it clear that relaxin can no longer be considered strictly a hormone of pregnancy. It has been shown that relaxin plays a physiological role on sperm motility. This book includes the study investigated at first that relaxin play a vital role not only on sperm motility but also on capacitation and acrosome reaction. This book also includes intracellular signaling cascades induced by relaxin in the stimulation of capacitation and acrosome reaction. We believe that the time has come to compile the book. It aims at confronting the different sophisticated approaches developed independently in different academics, human and animal reproduction laboratories and artificial insemination centers in the world. Specially, the first author gratefully acknowledges the assistance of his Postdoctoral host researcher Prof. Dr. Karl Schellander, and also grateful on Alexander von Humboldt Foundation for granting Postdoctoral Research Fellowship during the partial study in Germany.