|The present special issue of Metabolism is dedicated to Reproductive Endocrinology. The endocrinology of the gonads and the placenta constitutes the most rapidly evolving topic in Endocrinology and Metabolism, with many practical applications, especially in the field of human reproduction. Metabolism during the last two years has repeatedly published key papers in relevant topics, such as polycystic ovary syndrome (PCOS) [1–۵], vitamin D deficiency [6–۱۰], and pregnancy [11,12]. Nevertheless, a collection of papers that highlight the current diagnostic and therapeutic trends in Reproductive Endocrinology has not been attempted until today. When C.M. Mantzoros, editor-in-chief of Metabolism, had the idea of this special issue, the editorial team had to confront two main concerns. The first was the selection of the topics. We decided to include all main fields of Reproductive Endocrinology, namely “Reproductive Physiology”, “Female Reproductive Endocrinology”, “Male Reproductive Endocrinology”, “Endocrinology of pregnancy” and “Pediatric Reproductive Endocrinology”, in an attempt to cover every possible aspect (basic, translational, clinical) and age (embryo, child, adolescent, adult and aging). We had also to take under consideration different tissues (testis, ovary, adipose tissue, placenta), special situations (pregnancy), prevalent diseases (PCOS, hypovitaminosis D, thyroid disease, metabolic syndrome, hypogonadism, hypertension), special procedures (ovarian stimulation) and different treatments (testosterone replacement), not ignoring environmental issues (endocrine disruptors) and transitional ages (Kallmann, Klinefelter, and Turner syndromes during adolescent transition). The second was the selection of the authors. We find it exciting to form “mentor – mentee” pairs, where senior scientists with overwhelming contribution to their special fields of research would join forces with younger colleagues, who already have proved their research interest. The main idea was this Metabolism special issue to have an educational aspect for the younger colleagues, supporting the development of their career.