Publisher Summary This chapter explores the physiology, history, and key features of mechanical ventilators. A mechanical ventilator is an instrument that replaces, or assists in, spontaneous breathing. Mechanical ventilators are used in a variety of settings, from the operating room (OR) and intensive care unit (ICU) to the home and transport vehicles. A mechanical ventilator is an instrument that replaces or assists in spontaneous breathing. Mechanical ventilators are used in a variety of settings, from the operating room and intensive care unit to the home and transport vehicles. ICU ventilators are used by adult patients recovering from acute lung injuries. When a patient receives mechanical ventilation, this therapy may be based on pressure, volume, or flow control. Because flow is the derivative of volume and because the three variables are related by this equation, only one variable acts as the independent variable for control. After the control variable is chosen, the other two follow. Early ventilators were based on negative pressure, but are currently based on positive pressure. Key ventilator features include protection from interruption of power supply, maximum pressure to patient, active pressure limit, oxygen monitoring, and protection from breathing system leakage.