Physicians, or medical doctors, are highly trained health care providers who diagnose illnesses and prescribe medication. Their understanding of the functions of the human body requires an advanced comprehension of chemistry, biology, and human anatomy. Every doctor must attend medical school, an extremely intense 4 year program, which enhances their understanding of diseases, hospital equipment, and the ethics of the health care profession. After receiving an undergraduate degree, and a medical degree, newly graduated medical practitioners are often required to undertake a period of supervised practice before full registration is granted. This time period is called residency and often lasts between 1 and 2 years. Doctors are highly respected for their selflessness and sincere concern for the physical and mental welfare of others. The medical field is constantly developing and advancing, which makes the study of medicine a lifelong discipline.