Within the United States, the term physician also describes holders of the Doctor of Osteopathic medicine (D.O.) degree. However, outside the United States, osteopaths are not physicians who practice medicine. For further information on osteopathic medicine, see the entry on the comparison of MD and DO in the US.

Pediatricians must undertake further training in their chosen field. This may take from three to six or more years, depending on jurisdiction and the degree of specialization. The post-graduate training for a primary care physician, including primary care pediatricians, is generally not as lengthy as for a hospital-based medical specialist.

In most jurisdictions, entry-level degrees are common to all branches of the medical profession, but in some jurisdictions, specialization in pediatrics may begin before completion of this degree. In some jurisdictions, pediatric training is begun immediately following completion of entry-level training. In other jurisdictions, junior medical doctors must undertake generalist (unstreamed) training for a number of years before commencing pediatric (or any other) specialization. Specialist training is often largely under the control of pediatric organizations (see below) rather than universities, with varying degrees of government input, depending on jurisdiction.

"Pediatrician" versus "Paediatrician"

There is a slight semantic difference associated with the difference in spelling. In the USA, a pediatrician (US spelling) is a specialist physician who generally functions in a primary care setting for children. Like all physicians, they first receive a general medical degree (from a US medical school, typically MD or DO). Next, such pediatricians (US spelling) must complete an internship and then a 3-year residency in pediatrics. A similar situation exists in Germany: a kinderarzt is commonly a primary care pediatrician.

In the UK and the Commonwealth (and also in much of the rest of the world), a paediatrician is also a specialist physician for children, but generally not in primary care. He or she sees children who are either urgently taken to a hospital or who are referred by general practitioners; the latter see the bulk of child patients in primary care. Such paediatricians (British spelling) generally first receive a general medical degree (in the UK or Commonwealth, typically MB BS, MB BChir etc). Afterwards (in the UK or Commonwealth), they complete at least 2 years general clinical training ("foundation training"), then 6 or more years additional training in pediatrics or its subspecialties. A paediatrician in this sense could fairly be described as an internist who has subspecialized in infants & children.

Subspecialists in pediatrics

Specialist pediatricians may undergo further training in sub-specialties. Practising a subspecialty in pediatrics is similar in some respects to practising the relevant adult specialty, but a major difference is in the pattern of disease. Typically, diseases commonly seen in children are rare in adults (