Steroid signaling involves specific receptors that mediate genomic effects and many further proteins responsible for fast nongenomic activities. Metabolism at the position 17 of the steroid scaffold plays a pivotal role in the final regulation of the biological potency of steroid hormones. Enzymes responsible for that, the 17β-hydroxysteroid dehydrogenases (17β-HSD), act as carbonyl reductases and require cofactors for their catalytic activity. There is a substantial amount of evidence that human 17β-HSDs are as well involved in the metabolic pathways of retinoids and fatty acid beyond that which has so far been anticipated. At present fourteen 17β-HSDs have been annotated and characterized, and more might follow. Many of 17β-HSDs have been shown to be involved in the pathogenesis of human disorders and are targets for therapeutic intervention. Strategies on deciphering the physiological role of the 17β-HSD and the genetic predisposition for associated diseases will be presented involving analyses of animal models.