Abnormalities of respiratory mechanics are commonly present in patients who require mechanical ventilation. Monitoring and evaluating respiratory mechanics is the first step in mechanical ventilation. An appropriate manipulation of inspiratory pressure, expiratory pressure, inspiratory flow, tidal volume, and cycling, as well as patient?s respiratory effort, lung volume at end of inspiration and expiration, should be adjusted according to the patient?s respiratory mechanic abnormalities. ARDS is characterized of severe oxygenation disorder. The main targets of mechanical ventilation for ARDS patients are to open the lung and to keep the lung open, thus improving oxygenation while avoiding the over-distension of the lungs and ventilator induced lung injury (VILI). Chronic obstructive pulmonary disease (COPD) is characterized of expiratory flow restriction. The main targets of mechanical ventilation for COPD patients are to ensure adequate ventilation while avoiding dynamic hyperinflation and increase of intrinsic PEEP, as well as keeping respiratory drive appropriate. Monitoring respiratory mechanics in a daily clinical practice is crucially important for optimization of mechanical ventilation.