ABC and its variations are initialism mnemonics for essential steps used by both medical professionals and lay persons (such as first aiders) when dealing with a patient. In its original form it stands for Airway, Breathing, and Circulation. The protocol was originally developed as a memory aid for rescuers performing cardiopulmonary resuscitation, and the most widely known use of the initialism is in the care of the unconscious or unresponsive patient, although it is also used as a reminder of the priorities for assessment and treatment of patients in many acute medical and trauma situations, from first-aid to hospital medical treatment. Airway, breathing, and circulation are all vital for life, and each is required, in that order, for the next to be effective: a viable Airway is necessary for Breathing to provide oxygenated blood for Circulation. Since its development, the mnemonic has been extended and modified to fit the different areas in which it is used, with different versions changing the meaning of letters (such as from the original 'Circulation' to 'Compressions') or adding other letters (such as an optional "D" step for Disability or Defibrillation).

In 2010, the American Heart Association and International Liaison Committee on Resuscitation changed the recommended order of CPR interventions for most cases of cardiac arrest to chest compressions, airway, and breathing, or CAB.

At all levels of care, the ABC protocol exists to remind the person delivering treatment of the importance of airway, breathing, and circulation to the maintenance of a patient's life. These three issues are paramount in any treatment, in that the loss (or loss of control of) any one of these items will rapidly lead to the patient's death. The three objectives are so important to successful patient care that they form the foundation of training for not only first aid providers but also participants in many advanced medical training programs.

Hypoxia, the result of insufficient oxygen in the blood, is a potentially deadly condition and one of the leading causes of cardiac arrest. Cardiac arrest is the ultimate cause of clinical death for all animals (although with advanced intervention, such as cardiopulmonary bypass a cardiac arrest may not necessarily lead to death), and it is linked to an absence of circulation in the body, for any one of a number of reasons. For this reason, maintaining circulation is vital to moving oxygen to the tissues and carbon dioxide out of the body.

Airway, breathing, and circulation, therefore work in a cascade; if the patient's airway is blocked, breathing will not be possible, and oxygen cannot reach the lungs and be transported around the body in the blood, which will result in hypoxia and cardiac arrest. Ensuring a clear airway is therefore the first step in treating any patient; once it is established that a patient's airway is clear, rescuers must evaluate a patient's breathing, as many other things besides a blockage of the airway could lead to an absence of breathing.

The basic application of the ABC principle is in first aid, and is used in cases of unconscious patients to start treatment and assess the need for, and then potentially deliver, cardiopulmonary resuscitation.

In this simple usage, the rescuer is required to open the airway (using a technique such as "head tilt - chin lift"), then check for normal breathing. These two steps should provide the initial assessment of whether the patient will require CPR or not.

In the event that the patient is not breathing normally, the current international guidelines (set by the International Liaison Committee on Resuscitation or ILCOR) indicate that chest compressions should be started.