A fellowship is a formal, full-time training program that focuses on a particular area within the specialty, with requirements beyond the related residency.[4] Many highly specialized fields require formal training beyond residency. Examples of these include cardiology, endocrinology, oncology after internal medicine; cardiothoracic anesthesiology after anesthesiology; cardiothoracic surgery, pediatric surgery, surgical oncology after general surgery; reproductive endocrinology/infertility, maternal-fetal medicine, gynecologic oncology after obstetrics/gynecology. There are many others for each field of study.