 Asthma and COPD are characterized by an accelerated decline in lung function associated with incompletely reversible airflow obstruction. This could be the result of lung structural changes and inflammation. Tissue repairing mechanisms may result in a restitution ad integrum of bronchial epithelium. But in most cases, especially in COPD and severe asthma, the remodeling is characterized by mucus cells hyperplasia, overproduction of mucus, and physicochemical, biological and immunological changes. Clinically, this mucus overproduction is reported by patients as the clinical symptom called chronic bronchitis. Generally, it develops at a bronchiolar level where it is responsible for the progression of these diseases. There is a paradox, because the intrinsic properties of mucus seem rather beneficial so fighting against it may not be really wise at long-term. Especially its defensive effect against microbial agents which remains poorly explained. Currently, no treatment aims to reduce the production of mucus and mechanisms leading to such an overproduction are poorly understood in severe asthma and COPD. The identification of new targets to treat this overproduction of mucus in COPD is therefore of major interest. 