Reproductive endocrinology and infertility (REI) is a surgical subspecialty of obstetrics and gynecology that trains physicians in reproductive medicine addressing hormonal functioning as it pertains to reproduction as well as the issue of infertility. While most REI specialists primarily focus on the treatment of infertility, reproductive endocrinologists are trained to also evaluate and treat hormonal dysfunctions in females and males outside infertility. Reproductive endocrinologists have specialty training in obstetrics and gynecology (ob-gyn) before they undergo sub-specialty training (fellowship) in REI.