Oxitocin and vasopressin are substances of major relevance in the female endocrine and reproductive systems. They are involved in the control of several fundamental functions: luteolysis, myometrial contraction, labor and delivery of the fetus and placenta, post-partum uterine involution, milk ejection and infant lactation, blood pressure and kidney-dependent water balance regulation. Oxitocin and vasopressin were identified more than fifty years ago and are classically defined as neurohormones with, at least to some extent, overlapping actions. The recent developments of knowledge on oxytocin and vasopressin are changing our understanding of the roles played by these substances and even their actual nature. This is particularly true for oxytocin which at present must be considered no longer only a hormone; rather it is a much more complex substance with endocrine, paracrine and probably also autocrine properties. In this chapter, the contemporary roles and functions of oxytocin and vasopressin will be discussed with specific application to human pregnancy and parturition.