This review focuses on the expression and regulation of 3-hydroxysteroid dehydrogenase/(5)-(4) isomerase (3-HSD), with emphasis on the porcine version. 3-HSD is often associated with steroidogenesis, but its function in the metabolism of both steroids and xenobiotics is more obscure. Based on currently available literature covering humans, rodents and pigs, this review provides an overview of the present knowledge concerning the regulatory mechanisms for 3-HSD at all omic levels. The HSD isoenzymes are essential in steroid hormone metabolism, both in the synthesis and degradation of steroids. They display tissue-specific expression and factors influencing their activity, which therefore indicates their tissue-specific responses. 3-HSD is involved in the synthesis of a number of natural steroid hormones, including progesterone and testosterone, and the hepatic degradation of the pheromone androstenone. In general, a number of signaling and regulatory pathways have been demonstrated to influence 3-HSD transcription and activity, e.g., JAK-STAT, LH/hCG, ER, AR, SF-1 and PPAR. The expression and enzymic activity of 3-HSD are also influenced by external factors, such as dietary composition. Much of the research conducted on porcine 3-HSD is motivated by its importance for the occurrence of the boar taint phenomenon that results from high concentrations of steroids such as androstenone. This topic is also examined in this review.