Cytochrome P-450 aromatase (P450arom), the key enzyme for estrogen biosynthesis, is encoded by a single gene, namely the CYP19 gene, localized on 15q21.2. The human CYP19 gene spans about 123 kb with a coding region of 9 exons (about 30 kb, exon II–exon X). Although there are a number of alternative first exons and nine different transcriptional start sides with individual promoters that permit tissue-specific regulation of expression, the protein expressed in these various tissue sites (placenta, adipose tissue, brain, bone, ovary, etc.) is the same regardless of the promoter used. P450arom catalyzes the conversion of testosterone to estradiol, of androstenedione to estrone, and of 16α-hydroxylated dehydroepiandrosterone to estriol. As not only androgens but also estrogens are of importance, particularly in the male pubertal development, including bone changes which were classically considered mostly androgen dependent, the features of the aromatase deficiency syndrome in affected boys and girls as well as adult males and females are discussed. There is growing awareness that androgens and estrogens have general metabolic roles that reach far beyond reproductive processes. For instance, estrogen has a significant impact on carbohydrate and lipid metabolism, vascular function, and arteriosclerosis. In addition, extragonadal estrogen biosynthesis plays an important but often underestimated physiological and pathophysiological role, for example in breast cancer and endometriosis. Based on that knowledge, progress has been made as far as treatment and follow-up of these disorders are concerned. In addition, there is a focus on the treatment of children suffering from a lack of P450arom activity.