Searching for the cause of gender-variant behavior has been a preoccupation of the clinical and medical community since sexologists first began presenting case material about inverts in the late nineteenth century. Indeed, since their goal was to cure or treat gender dysphoria and/or to assist intersex people in achieving complete psychosexual congruity to an assigned gender, understanding the etiology of gender development and cross-gender behavior logically became the prevailing focal point of their research. Sexologists, researchers, and clinicians have classified and categorized various “types” of gender variance according to presentation, severity of dysphoria, the eroticism of the cross-dressing, and the person’s sexual desires. In this chapter many typologies will be explored that have been developed to assist in a clinical understanding of gender identity concerns.