3 Chronic obstructive pulmonary disease (COPD) is a major global health problem due to 26 its high prevalence (about 10% of the adult population), raising incidence (related in 27 part to the ageing of the population) and very significant associated personal, social and 28 economic costs (1). The World COPD Day (November 18, 2020) is organized by the 29 Global Initiative for Chronic Obstructive Lung Disease (GOLD) in collaboration with 30 health care professionals and COPD patient groups throughout the world to raise 31 awareness about COPD and to improve COPD care throughout the world (1). To 32 contribute to this goal, here we review the main changes occurred over the past few 33 years in our understanding of COPD in order to stimulate further research in the 34 challenges that this new knowledge opens. 35