An immense plethora of translational knowledge has been acquired since the first description of ARDS in 1967. However, estimates of the incidence of ARDS and ALI have varied widely, and the true magnitude of this health problem still remains unclear. Current estimates of the incidence of ALI/ARDS range from 15 to 80 cases per 100.000 population, or almost 40.000 cases per year in Spain. Combined mortality rates for ALI/ARDS range between 30-45%. ALI and ARDS occur as a complication or as the primary cause of critical illness in patients, usually after severe infection or trauma.