Emergency medicine, also known as accident and emergency medicine, is the medical specialty concerned with the care of illnesses or injuries requiring immediate medical attention. Emergency physicians care for unscheduled and undifferentiated patients of all ages. As first-line providers, their primary responsibility is to initiate resuscitation and stabilization and to start investigations and interventions to diagnose and treat illnesses in the acute phase. Emergency physicians generally practice in hospital emergency departments, pre-hospital settings via emergency medical services, and intensive care units, but may also work in primary care settings such as urgent care clinics. Sub-specializations of emergency medicine include disaster medicine, medical toxicology, ultrasonography, critical care medicine, hyperbaric medicine, sports medicine, palliative care, or aerospace medicine.