First aid is the first and immediate assistance given to any person with a medical emergency, with care provided to preserve life, prevent the condition from worsening, or to promote recovery until medical services arrive. First aid is generally performed by someone with basic medical or first response training. Mental health first aid is an extension of the concept of first aid to cover mental health, while psychological first aid is used as early treatment of people who are at risk for developing PTSD. Conflict first aid, focused on preservation and recovery of an individual's social or relationship well-being, is being piloted in Canada.

There are many situations that may require first aid, and many countries have legislation, regulation, or guidance, which specifies a minimum level of first aid provision in certain circumstances. This can include specific training or equipment to be available in the workplace (such as an automated external defibrillator), the provision of specialist first aid cover at public gatherings, or mandatory first aid training within schools. Generally, five steps are associated with first aid:

Skills of what is now known as first aid have been recorded throughout history, especially in relation to warfare, where the care of both traumatic and medical cases is required in particularly large numbers. The bandaging of battle wounds is shown on Classical Greek pottery from c. 500 BC, whilst the parable of the Good Samaritan includes references to binding or dressing wounds. There are numerous references to first aid performed within the Roman army, with a system of first aid supported by surgeons, field ambulances, and hospitals. Roman legions had the specific role of capsarii, who were responsible for first aid such as bandaging, and are the forerunners of the modern combat me.

Further examples occur through history, still mostly related to battle, with examples such as the Knights Hospitaller in the 11th century AD, providing care to pilgrims and knights in the Holy Land.

During the late 18th century, drowning as a cause of death was a major concern amongst the population. In 1767, a society for the preservation of life from accidents in water was started in Amsterdam, and in 1773, physician William Hawes began publicizing the power of artificial respiration as means of resuscitation of those who appeared drowned. This led to the formation, in 1774, of the Society for the Recovery of Persons Apparently Drowned, later the Royal Humane Society, who did much to promote resuscitation.

Napoleon's surgeon, Baron Dominique-Jean Larrey, is credited with creating an ambulance corps, the ambulance volantes, which included medical assistants, tasked to administer first aid in battle.

In 1859, Swiss businessman Jean-Henri Dunant witnessed the aftermath of the Battle of Solferino, and his work led to the formation of the Red Cross, with a key stated aim of "aid to sick and wounded soldiers in the field". The Red Cross and Red Crescent are still the largest provider of first aid worldwide.

In 1870, Prussian military surgeon Friedrich von Esmarch introduced formalized first aid to the military, and first coined the term "erste hilfe" (translating to 'first aid'), including training for soldiers in the Franco-Prussian War on care for wounded comrades using pre-learnt bandaging and splinting skills, and making use of the Esmarch bandage which he designed. The bandage was issued as standard to the Prussian combatants, and also included aide-memoire pictures showing common uses.