A bag-valve mask (BVM) is a medical device used to provide positive pressure ventilation to individuals who are not breathing or not breathing adequately. It consists of a self-expanding bag, a unidirectional valve, and a facial mask. The mask is placed over the patient's nose and mouth, allowing the rescuer to deliver air or oxygen into the lungs by squeezing the bag. BVMs are commonly used in emergency situations, such as cardiac arrest or respiratory failure, by trained professionals like paramedics and emergency medical technicians. They are essential tools in advanced airway management and can help maintain oxygen levels until further medical assistance is available.