An injury is any physiological damage to living tissue caused by immediate physical stress. Injuries to humans can occur intentionally or unintentionally and may be caused by blunt trauma, penetrating trauma, burning, toxic exposure, asphyxiation, or overexertion. Injuries can occur in any part of the body, and different symptoms are associated with different injuries.

Treatment of a major injury is typically carried out by a health professional and varies greatly depending on the nature of the injury. Traffic collisions are the most common cause of accidental injury and injury-related death among humans. Injuries are distinct from chronic conditions, psychological trauma, infections, or medical procedures, though injury can be a contributing factor to any of these.

Several major health organizations have established systems for the classification and description of human injuries.

Injuries may be intentional or unintentional. Intentional injuries may be acts of violence against others or self-inflicted against one's own person. Accidental injuries may be unforeseeable, or they may be caused by negligence. In order, the most common types of unintentional injuries are traffic accidents, falls, drowning, burns, and accidental poisoning. Certain types of injuries are more common in developed countries or developing countries. Traffic injuries are more likely to kill pedestrians than drivers in developing countries. Scalding burns are more common in developed countries, while open-flame injuries are more common in developing countries.^{[clarification needed]}

As of 2021, approximately 4.4 million people are killed due to injuries each year worldwide, constituting nearly 8% of all deaths. 3.16 million of these injuries are unintentional, and 1.25 million are intentional. Traffic accidents are the most common form of deadly injury, causing about one-third of injury-related deaths. One-sixth are caused by suicide, and one-tenth are caused by homicide. Tens of millions of individuals require medical treatment for nonfatal injuries each year, and injuries are responsible for about 10% of all years lived with disability. Men are twice as likely to be killed through injury than women. In 2013, 367,000 children under the age of five died from injuries, down from 766,000 in 1990.

The World Health Organization (WHO) developed the International Classification of External Causes of Injury (ICECI). Under this system, injuries are classified by mechanism of injury, objects/substances producing injury, place of occurrence, activity when injured, the role of human intent, and additional modules. These codes allow the identification of distributions of injuries in specific populations and case identification for more detailed research on causes and preventive efforts.

The United States Bureau of Labor Statistics developed the Occupational Injury and Illness Classification System (OIICS). Under this system injuries are classified by nature, part of body affected, source and secondary source, and event or exposure. The OIICS was first published in 1992 and has been updated several times since. The Orchard Sports Injury and Illness Classification System (OSIICS), previously OSICS, is used to classify injuries to enable research into specific sports injuries.

The injury severity score (ISS) is a medical score to assess trauma severity. It correlates with mortality, morbidity, and hospitalization time after trauma. It is used to define the term major trauma (polytrauma), recognized when the ISS is greater than 15. The AIS Committee of the Association for the Advancement of Automotive Medicine designed and updates the scale.