Pediatrics (also spelled paediatrics or pædiatrics) is the branch of medicine that involves the medical care of infants, children, and adolescents. The American Academy of Pediatrics recommends people be under pediatric care up to the age of 21.  A medical doctor who specializes in this area is known as a pediatrician, or paediatrician. The word pediatrics and its cognates mean "healer of children"; they derive from two Greek words: παῖς (pais "child") and ἰατρός (iatros "doctor, healer"). Pediatricians work both in hospitals, particularly those working in its subspecialties such as neonatology, and as primary care physicians.
Already Hippocrates, Aristotle, Celsus, Soranus, and Galen understood the differences in growing and maturing organisms that necessitated different treatment: Ex toto non sic pueri ut viri curari debent ("In general, boys should not be treated in the same way as men." Celsus). 
Some of the oldest traces of pediatrics can be discovered in Ancient India where children's doctors were called kumara bhrtya.  Sushruta Samhita an ayurvedic text, composed during the sixth century BC contains the text about pediatrics. Another ayurvedic text from this period is Kashyapa Samhita.
A second century AD manuscript by the Greek physician and gynecologist Soranus of Ephesus dealt with neonatal pediatrics.  Byzantine physicians Oribasius, Aëtius of Amida, Alexander Trallianus, and Paulus Aegineta contributed to the field.  The Byzantines also built brephotrophia (crêches).  Islamic writers served as a bridge for Greco-Roman and Byzantine medicine and added ideas of their own, especially Haly Abbas, Serapion, Avicenna, and Averroes. The Persian scholar and doctor al-Razi (865–925) published a short treatise on diseases among children.  The first book about pediatrics was Libellus [Opusculum] de aegritudinibus et remediis infantium 1472 ("Little Book on Children Diseases and Treatment"), by the Italian pediatrician Paolo Bagellardo.  In sequence came Bartholomäus Metlinger's Ein Regiment der Jungerkinder 1473, Cornelius Roelans (1450–1525) no title Buchlein, or Latin compendium, 1483, and Heinrich von Louffenburg (1391–1460) Versehung des Leibs written in 1429 (published 1491), together form the Pediatric Incunabula, four great medical treatises on children's physiology and pathology. 
The Swedish physician Nils Rosén von Rosenstein (1706–1773) is considered to be the founder of modern pediatrics as a medical specialty,   while his work The diseases of children, and their remedies (1764) is considered to be "the first modern textbook on the subject".  Pediatrics as a specialized field of medicine continued to develop in the mid-19th century; German physician Abraham Jacobi (1830–1919) is known as the father of American pediatrics because of his many contributions to the field.   He received his medical training in Germany but later practiced in New York City.
The first generally accepted pediatric hospital is the Hôpital des Enfants Malades (French: Hospital for Sick Children ), which opened in Paris in June 1802 on the site of a previous orphanage. From its beginning, this famous hospital accepted patients up to the age of fifteen years,  and it continues to this day as the pediatric division of the Necker-Enfants Malades Hospital, created in 1920 by merging with the physically contiguous Necker Hospital, founded in 1778.
In other European countries, the Charité (a hospital founded in 1710) in Berlin established a separate Pediatric Pavilion in 1830, followed by similar institutions at Sankt Petersburg in 1834, and at Vienna and Breslau (now Wrocław), both in 1837. In 1852 Britain's first pediatric hospital, the Hospital for Sick Children, Great Ormond Streets. The first Children's hospital in Scotland opened in 1860 in Edinburgh.  In the US, the first similar institutions were the Children's Hospital of Philadelphia, which opened in 1855, and then Boston Children's Hospital (1869). Subspecialties in pediatrics were created at the Harriet Lane Home at Johns Hopkins Hospital by Edwards A. Park. 
Differences between adult and pediatric medicine
The body size differences are paralleled by maturation changes. The smaller body of an infant or neonate is substantially different physiologically from that of an adult. Congenital defects, genetic variance, and developmental issues are of greater concern to pediatricians than they often are to adult physicians. A common adage is that children are not simply "little adults". The clinician must take into account the immature physiology of the infant or child when considering symptoms, prescribing medications, and diagnosing illnesses.
A major difference between the practice of pediatric and adult medicine is that children, in most jurisdictions and with certain exceptions, cannot make decisions for themselves. The issues of guardianship, privacy, legal responsibility and informed consent must always be considered in every pediatric procedure. Pediatricians often have to treat the parents and sometimes, the family, rather than just the child. Adolescents are in their own legal class, having rights to their own health care decisions in certain circumstances. The concept of legal consent combined with the non-legal consent (assent) of the child when considering treatment options, especially in the face of conditions with poor prognosis or complicated and painful procedures/surgeries, means the pediatrician must take in to account the desires of many people, in addition to those of the patient.
Aspiring medical students will need 4 years of undergraduate courses at a college or university, which will get them a BS, BA, or other bachelor's degree. After completing college future pediatricians will need to attend 4 years of medical school and later do 3 more years of residency training, the first year of residency is formerly called "Internship." After completing the 3 years of residency, physicians are eligible to become certified in pediatrics by passing a rigorous test that deals with medical conditions related to young children.
In high school, future pediatricians are required to take basic science classes, such as, biology, chemistry, physics, algebra, geometry, and calculus and also foreign language class, preferably Spanish, and get involved in high school organizations and extracurricular activities. After high school, college students simply need to fulfill the basic science course requirements that most medical schools recommend and will need to prepare to take the MCAT (Medical College Admission Test) their junior or early senior year in college. Once attending medical school, students courses will focus on basic medical sciences, like human anatomy, physiology, chemistry, etc., for the first three years. The second year is when medical students start to get "hands-on" experience with patients. 
Training of pediatricians
The training of pediatricians varies considerably across the world. Depending on jurisdiction and university, a medical degree course may be either undergraduate-entry or graduate-entry. The former commonly takes five or six years, and has been usual in the Commonwealth. Entrants to graduate-entry courses (as in the US), usually lasting four or five years, have previously completed a three- or four-year university degree, commonly but by no means always in sciences. Medical graduates hold a degree specific to the country and university in and from which they graduated. This degree qualifies that medical practitioner to become licensed or registered under the laws of that particular country, and sometimes of several countries, subject to requirements for " internship " or "conditional registration".
Pediatricians must undertake further training in their chosen field. This may take from four to eleven or more years, (depending on jurisdiction and the degree of specialization).
In the United States, a medical school graduate wishing to specialize in pediatrics must undergo a three-year residency composed of outpatient, inpatient, surgical, and critical care rotations. Specialties within pediatrics require further training in the form of 3-year fellowships. Specialties include critical care, gastroenterology, neurology, infectious disease, hematology/oncology, rheumatology, pulmonology, child abuse, emergency medicine, endocrinology, neonatology, and others. 
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, and depend on jurisdiction.
Subspecialties of pediatrics include:
(not an exhaustive list)
- Adolescent medicine
- Child abuse pediatrics
- Clinical informatics
- Developmental-behavioral pediatrics
- Headache medicine
- Hospice & palliative care
- Pain medicine
- Pediatric allergy and immunology
- Pediatric cardiology
- Pediatric critical care
- Pediatric emergency medicine
- Pediatric endocrinology
- Pediatric gastroenterology
- Pediatric hematology
- Pediatric infectious disease
- Pediatric nephrology
- Pediatric neuropsychology
- Pediatric oncology
- Pediatric neuro-oncology
- Pediatric pulmonology
- Pediatric rheumatology
- Sleep medicine
- Social pediatrics
- Sports medicine
- Transplant hepatology
Other specialties that care for children include:
- Child neurology, a specialty in its own right
- Neurocritical Care
- Pediatric neuro-oncology
- Child psychiatry, subspecialty of psychiatry
- Pediatric anesthesiology, subspecialty of anesthesiology
- Pediatric dermatology, subspecialty of dermatology
- Pediatric neurosurgery, subspecialty of neurosurgery
- Pediatric ophthalmology, subspecialty of ophthalmology
- Pediatric orthopedic surgery, subspecialty of orthopedic surgery
- Pediatric otolaryngology, subspecialty of otolaryngology
- Pediatric rehabilitation medicine, subspecialty of physical medicine and rehabilitation
- Pediatric surgery, subspecialty of general surgery
- Pediatric urology, subspecialty of urology