To perform mouth-to-mouth resuscitation on an injured person, what must you do first?

To perform mouth-to-mouth resuscitation, the first step is to ensure that the injured person's airway is clear, meaning that the passages allowing air to flow from the mouth and nose to the lungs are not obstructed. This can be done with the head-tilt chin-lift maneuver: with one hand we slightly lift the chin and with the other we gently push the forehead back. The most common cause of airway obstruction is the tongue falling backwards.