A physician (also called doctor in some places) is a person who practices medicine. In the United States, the term physician is traditional and commonly used. In Britain & Ireland, Canada, Australia, New Zealand, Japan, South Africa, India, Indonesia, Pakistan, Bangladesh, Sri Lanka and Zimbabwe, the term doctor is more common (and in formal/legal contexts, medical practitioner as well), as physician refers to specialists in internal medicine.

Because of the extensive training requirements, physicians are traditionally considered to be members of a learned profession.

In all developed countries, entry-level medical education programs are tertiary-level courses undertaken at a medical school pertaining to a university. Depending on jurisdiction and university, these may be either undergraduate-entry or graduate-entry programs. Following completion of entry-level training, newly graduated doctors are often required to undertake a period of supervised practice before full registration is granted; this may be referred to as "internship" or "conditional registration".

Further training in a particular field of medicine may be undertaken. In some jurisdictions this is commenced immediately following completion of entry-level training, whilst other jurisdictions require junior doctors to undertake generalist (un-streamed) training for a number of years before commencing specialization. Various teaching methodologies have been utilized in medical education, which is an active area of educational research.

In the USA, there are two types of physicians. Allopathic physicians hold a MD and osteopathic physicians hold a DO. Both physicians are fully licensed to practice in all 50 states. In most jurisdictions, physicians need government permission to practice. This is known as licensing in the United States. Regulating authorities will revoke permission to practice in cases of malpractice or serious misconduct.

Graduates of Foreign Medical Schools, who enter USA have to pass USMLE step 1 and 2 ECFMG and do a residency program to qualify for a state license. After graduating from medical school, American physicians usually take a standardized exam which enables them to obtain a certificate to practice from the appropriate state agency. All American states have an agency which is usually called the "Medical Board," although there are alternate names such as "Board of Medicine," "Board of Medical Examiners," "Board of Medical Licensure," "Board of Healing Arts," etc. Australian states usually have a "Medical Board," while Canadian provinces usually have a "College of Physicians and Surgeons."

In the United States, as a result of the war on drugs, pharmaceuticals are strictly regulated at the federal level by the Food and Drug Administration and the Drug Enforcement Administration. All practicing American physicians who intend to prescribe controlled substances must obtain a number from the DEA, and that DEA number must appear on all their prescriptions. Use of the DEA number enables dispensing pharmacists or the DEA to ensure that a physician is not dispensing potentially addictive or harmful drugs, such as opiates or stimulants, in contravention to accepted standards of care.

Medical care is shared between the medical profession (physicians or doctors) and other professionals such as nurses and pharmacists, sometimes known as allied health professionals. Historically, only those with a medical doctorate have been considered to practice medicine. Clinicians (licensed professionals who deal with patients) can be physicians, nurses, therapists or others. The medical profession is the social and occupational structure of the group of people formally trained and authorized to apply medical knowledge. Many countries and legal jurisdictions have legal limitations on who may practice medicine.

Medicine comprises various specialized sub-branches, such as cardiology, pulmonology, neurology, or other fields such as sports medicine, research or public health.

Human societies have had various different systems of health care practice since at least the beginning of recorded history. Medicine, in the modern period, is the mainstream scientific tradition which developed in the Western world since the early Renaissance (around 1450). Many other traditions of health care are still practiced throughout the world; most of these are separate from Western medicine, which is also called biomedicine, allopathic medicine or the Hippocratic tradition. The most highly developed of these are traditional Chinese medicine and the Ayurvedic traditions of India and Sri Lanka. Various non-mainstream traditions of health care have also developed in the Western world. These systems are sometimes considered companions to Hippocratic medicine, and sometimes are seen as competition to the Western tradition. Few of them have any scientific confirmation of their tenets, because if they did they would be brought into the fold of Western medicine.

"Medicine" is also often used amongst medical professionals as shorthand for internal medicine. Veterinary medicine is the practice of health care in animal species other than human beings.

The earliest type of medicine in most cultures was the use of plants (Herbalism) and animal parts. This was usually in concert with 'magic' of various kinds in which animism (the notion of inanimate objects having spirits; or communion with ancestor spirits), shamanism (the vesting of an individual with mystic powers), and divination (the supposed obtaining of truth by magic means) played a major role.

The practice of medicine combines both science and art. Science and technology are the evidence base for many clinical problems for the general population at large. The art of medicine is the application of this medical knowledge in combination with intuition and clinical judgment to determine the proper diagnoses and treatment plan for each unique patient and to treat the patient accordingly.

Central to medicine is the patient-doctor relationship established when a person with a health concern or problem seeks the help of a physician (i.e. the medical encounter). Other health professionals similarly establish a relationship with a patient and may perform interventions from their perspective, e.g. nurses, radiographers and therapists.

Medical care delivery is classified into primary, secondary and tertiary care. Primary care medical services are provided by physicians or other health professionals who has first contact with a patient seeking medical treatment or care. These occur in physician's office, clinics, nursing homes, schools, home visits and other places close to patients. About 90% of medical visits can be treated by the primary care provider. These include treatment of acute and chronic illnesses, preventive care and health education for all ages and both sex.