**1. Introduction**

The term "sex-steroids" refers to estrogen, androgen and progestin products of vertebrate gonads. Sex steroids were so named for their influence on the sexually dimorphic development of the reproductive tract, secondary sex characters and central nervous system, which cause subsequent sexually dimorphic behavior and physiology (Phoenix et al. 1959b; Breedlove & Arnold 1983b). Receptors for sex-steroids are present in almost all tissues, and sex-steroids may be synthesized from cholesterol in the gonads, adrenals and brain. Although commonly described as endocrine components released into the bloodstream, sexsteroids may be generated through conversion from other (particularly adrenal) circulating steroids (Hinson et al. 2010) or generated de novo from cholesterol via intracrine pathways, as in the case of the brain neurosteroids (Baulieu 1997).

In this chapter we examine how sex-steroids fit into the larger themes of metazoan physiology and reproduction, and examine why these compounds may function the way they do in vertebrates. I aim to present broad concepts in a manner that is easily accessible to the nonspecialized reader. It will be useful for the reader to be able to navigate modern versions of metazoan systematics. Therefore I aim to utilize the open-source nature of this publication by providing links that encourage the reader to use the Tree of Life web project (http://tolweb.org/tree/) for up-to-date "locations" of animals within the organization of living things. Navigation instructions for the Tree of Life web project are located at: (http://tolweb.org/tree/home.pages/navigating.html). In summary, clicking the leftwardpointing arrow on a given tree will navigate to the next broader category. Clicking text on the right side of the tree will navigate inside the highlighted group (the next narrower category).
