The steroid hormones constitute a very important class of compounds and are present in most species. The hormones that are synthesized in the adrenal gland, namely the glucocorticoids and the mineralocorticoids, are important for the maintenance of carbohydrate metabolism, stress management, and salt balance. The “male” sex steroids, the androgens, which are responsible for the maintenance of reproductive function and secondary sex characteristics, are also an important member of this family and are synthesized in the testis. Lastly, the “female” sex steroids, the progestins and estrogens, are synthesized in the ovary and the placenta and function to induce secondary sex characteristics and are also essential for reproduction. An additional class of steroids, known as the neuroactive or neural steroids, are synthesized by the central nervous system (CNS) and appear to have specialized functions in this tissue. While neural steroids constitute an extremely interesting class of compounds, they will not be considered further in this review. Thus, the steroid hormones can be distinguished by their diverse physiological actions in the body. However, regardless of the tissue of origin, the steroid hormones are all synthesized from a common substrate, cholesterol; therefore, they are all structurally similar. The biosynthesis of all hormonal steroids in response to trophic hormone and other steroidogenic stimuli begins with the enzymatic cleavage of the side chain of cholesterol to form the first steroid synthesized in all steroidogenic tissues, pregnenolone.