Prior to any investigation of toxicant effects on sexual development it is necessary to have a complete understanding of the relevant physiology of reproductive development. Beginning at conception, development of males and females diverge to form the respective reproductive systems. From the prenatal period to the interval following puberty, radical changes take place in the hypothalamo-pituitary-gonadal axis of males and females. The complexity of each of these systems and their development is mirrored in the many possibilities for the means by which chemicals may produce adverse effects. For example, a chemical that affects hormone synthesis may, if administered at the proper time, affect hypothalamic development. As a consequence, pubertal development may not occur normally. In this chapter, we have outlined the basics of reproductive development and provided examples of adverse effects by endocrine disrupting chemicals (EDCs) on such development.