Mechanical ventilation or assisted ventilation is the medical term for using a ventilator machine to fully or partially provide artificial ventilation. Mechanical ventilation helps move air into and out of the lungs, with the main goal of helping the delivery of oxygen and removal of carbon dioxide. Mechanical ventilation is used for many reasons, including to protect the airway due to mechanical or neurologic cause, to ensure adequate oxygenation, or to remove excess carbon dioxide from the lungs. Various healthcare providers are involved with the use of mechanical ventilation and people who require ventilators are typically monitored in an intensive care unit.

Mechanical ventilation is termed invasive if it involves an instrument to create an airway that is placed inside the trachea. This is done through an endotracheal tube or nasotracheal tube. For non-invasive ventilation in people who are conscious, face or nasal masks are used. The two main types of mechanical ventilation include positive pressure ventilation where air is pushed into the lungs through the airways, and negative pressure ventilation where air is pulled into the lungs. There are many specific modes of mechanical ventilation, and their nomenclature has been revised over the decades as the technology has continually developed.

The Greek physician Galen may have been the first to describe mechanical ventilation: "If you take a dead animal and blow air through its larynx [through a reed], you will fill its bronchi and watch its lungs attain the greatest distention." In the 1600s, Robert Hooke conducted experiments on dogs to demonstrate this concept. Vesalius too describes ventilation by inserting a reed or cane into the trachea of animals. These experiments predate the discovery of oxygen and its role in respiration. In 1908, George Poe demonstrated his mechanical respirator by asphyxiating dogs and seemingly bringing them back to life. These experiments all demonstrate positive pressure ventilation.

To achieve negative pressure ventilation, there must be a sub-atmospheric pressure to draw air into the lungs. This was first achieved in the late 19th century when John Dalziel and Alfred Jones independently developed tank ventilators, in which ventilation was achieved by enclosing a patient's body within a chamber of sub-atmospheric pressure. This machine came to be known colloquially as the Iron lung, which went through many iterations of development. The use of the iron lung became widespread during the polio epidemic of the 1900s.

The first functional positive pressure ventilator was invented by Bjørn Aage Ibsen in Blegdamshospitalet, Copenhagen, Denmark in 1952. Denmark was experiencing a poliomyelitis epidemic, and there was only one iron lung in the hospital. The first patient to be saved by the new machine was 12-year old Vivi Ebert. Ibsen gave the patent on free use and licensed it to AGA, Sweden, to enable quick serial production of the new machines. The positive pressure ventilator quickly superseded the iron lung in Europe by 1953, but the older machine persisted in the United States.

Early ventilators were control style with no support breaths integrated into them and were limited to an inspiration to expiration ratio of 1:1. In the 1970s, intermittent mandatory ventilation was introduced as well as synchronized intermittent mandatory ventilation. These styles of ventilation had control breaths that patients could breathe between.

Mechanical ventilation is indicated when a patient's spontaneous breathing is inadequate to maintain life. It may be indicated in anticipation of imminent respiratory failure, acute respiratory failure, acute hypoxemia, or prophylactically. Because mechanical ventilation serves only to provide assistance for breathing and does not cure a disease, the patient's underlying condition should be identified and treated in order to liberate them from the ventilator.

Common specific medical indications for mechanical ventilation include: