Family 19 of the P450 super family is responsible for the conversion of C19 androgenic steroids to the corresponding estrogens, a reaction known as aromatization because it involves conversion of the delta4-3-one A-ring of the androgens to the corresponding phenolic A-ring characteristic of estrogens. The gene encoding human aromatase has been cloned and characterized and shown to be unusual compared to genes encoding other P450 enzymes, because there are numerous untranslated first exons that occur in aromatase transcripts in a tissue-specific fashion due to differential splicing as a consequence of the use of tissue-specific promoters. Thus, expression in the ovary uses a proximal promoter that is regulated primarily by cAMP. On the other hand, expression in the placenta uses a distal promoter located at least 40 kb upstream of the start of transcription that is regulated by retinoids. Other promoters are used in brain and adipose tissue. In the latter case, class I cytokines such as IL-6 and IL-11, as well as TNF-alpha, are important regulatory factors. A common 3'-splice junction located upstream of the start of translation is used in all of the splicing events involved in the use of these various promoters. Thus, the coding region of the transcripts, and hence the protein, are identical regardless of the tissue site of expression; what differs in a tissue-specific fashion is the 5'-end of the transcripts. This pattern of expression has great significance both from a phylogenetic and ontogenetic standpoint, as well as for the physiology and pathophysiology of estrogen formation, as will be discussed in this review.