After providing diagnoses, a doctor treats patients who are suffering from diseases and injuries. A doctor is also called a physician and may be either an M.D. (medical doctor) or D.O. (doctor of osteopathic medicine). M.D.s and D.O.s both use traditional treatment methods such as drugs and surgery, but D.O.s emphasize the body's musculoskeletal system, preventive medicine and holistic patient care. Doctors can be primary care physicians or they may specialize in a particular area of medicine such as obstetrics, pediatrics or anesthesiology.

There were 661,000 doctors employed in 2008.

To become a doctor one must spend four years in college earning a bachelor's degree. This must be followed by four years of medical school and then three to eight years of internship and residency depending on specialty.

To practice as a physician in the United States one must pass the United States Medical Licensing Examination (USMLE). To become board certified in a specialty, an M.D. or D.O. may have to spend up to seven years in residency training. Upon completion of that residency or after one to two years of practice, he or she will have to take a final examination in order to become certified by a member board of the American Board of Medical Specialists (ABMS) or the American Osteopathic Association (AOA). Certification in a subspecialty may require an additional one or two year residency.

Gaining experience in a specialty or subspecialty and developing a good reputation among patients and peers, can help a doctor advance in his or her career. Many start their own private practices or join already established ones. Other experienced doctors teach residents and new doctors while some take on managerial positions in hospitals or clinics.

According to the U.S. Bureau of Labor Statistics, employment of doctors is projected to grow much faster than the average for all occupations through 2018. The best opportunities will be in rural and low-income areas.

Primary care physicians earned a median annual salary of $186,044 in 2008, while specialists earned a median annual salary of $339,738.

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