Relaxin is a protein hormone, produced and secreted during pregnancy in mammalian species, having superficial structural features resembling those of insulin. Since its initial isolation from the ovaries of pregnant pigs in 1976, increasing interest in relaxin has led to increased understanding of the chemistry, synthesis, secretion, biological roles, mechanisms of action, and potential clinical applications of relaxin in humans and domestic animals. In pigs, rats, and mice, relaxin promotes growth and softening of the cervix, enabling rapid and safe delivery of the fetuses. In these species relaxin also promotes growth and development of the mammary apparatus. Recently, biological effects of relaxin in the heart, kidney, liver, and brain have been identified, and these discoveries have triggered additional interest in possible clinical applications for relaxin. In 2002, a second form of relaxin, which is found primarily in the brain, was discovered. Relaxin-like factor (also called insulin 3), which was discovered in 1993, is produced in the fetal testis and plays a major role in testicular descent during development. The recent identification of the receptors for both relaxin and relaxin-like factor has enabled more rigorous studies of the target tissues and mechanisms of action of these hormones. This volume contains a description of recent advances and future research and clinical possibilities in the field of relaxin and related peptides.