A medical specialty is a branch of medical practice that is focused on a defined group of patients, diseases, skills, or philosophy. Examples include those branches of medicine that deal exclusively with children (paediatrics), cancer (oncology), laboratory medicine (pathology), or primary care (family medicine). After completing medical school or other basic training, physicians or surgeons and other clinicians usually further their medical education in a specific specialty of medicine by completing a multiple-year residency to become a specialist. History of medical specialization To a certain extent, medical practitioners have long been specialized. According to Galen, specialization was common among Roman physicians. The particular system of modern medical specialties evolved gradually during the 19th century. Informal social recognition of medical specialization evolved before the formal legal system. The particular subdivision of the practice of medicine into various specialties varies from country to country, and is somewhat arbitrary. Classification of medical specialization Medical specialties can be classified along several axes. These are: Surgical or internal medicine Age range of patients Diagnostic or therapeutic Organ-based or technique-based Throughout history, the most important has been the division into surgical and internal medicine specialties. The surgical specialties are those in which an important part of diagnosis and treatment is achieved through major surgical techniques. The internal medicine specialties are the specialties in which the main diagnosis and treatment is never major surgery. In some countries, anesthesiology is classified as a surgical discipline, since it is vital in the surgical process, though anesthesiologists never perform major surgery themselves. Many specialties are organ-based. Many symptoms and diseases come from a particular organ. Others are based mainly around a set of techniques, such as radiology, which was originally based around X-rays. The age range of patients seen by any given specialist can be quite variable. Paediatricians handle most complaints and diseases in children that do not require surgery, and there are several subspecialties (formally or informally) in paediatrics that mimic the organ-based specialties in adults. Paediatric surgery may or may not be a separate specialty that handles some kinds of surgical complaints in children. A further subdivision is the diagnostic versus therapeutic specialties. While the diagnostic process is of great importance in all specialties, some specialists perform mainly or only diagnostic examinations, such as pathology, clinical neurophysiology, and radiology. This line is becoming somewhat blurred with interventional radiology, an evolving field that uses image expertise to perform minimally