During artificial ventilation the lungs are intermittently insufflated by respiratory gases in a certain pattern (ventilatory conditions). The consequences in terms of stretch of lung tissue, airway pressure, alveolar pressure and gas exchange depend on the mechanical properties of airways, lung tissue and thoracic wall. These properties are age dependent and may be changed considerably by disease. The basic principles of ventilatory support can be explained as an interaction of ventilatory gases passing through the airways in and out of the lungs, the subsequent changes in stretch of lung tissue and thoracic wall and pressure variations in the airways and alveoli. Overstretched lung and airways tissue causes damage (barotrauma, air leakage). During artificial ventilation intrathoracic pressure increases, which diminishes venous return to the heart and decreases cardiac output. In addition, the sometimes inevitably high oxygen concentrations may be toxic. In this article the ventilatory cycle is analysed into its different sequences and the interaction with the respiratory system and its consequences are discussed.