Summary: One of the main innate defense mechanisms of the respiratory system is the production of airway mucus which is responsible for the physical retention of harmful particles – pathogens, allergens, dusts. In the course of chronic diseases, e.g. asthma, COPD, bronchiectasis or cystic fibrosis, one of the symptoms is excessive production of this secretion. Due to this fact, it is worth considering their treatment with mucolytic agents that facilitate the clearing of the airways from residual secretions. The main pharmacological mechanism of these drugs is the degradation of disulphide bridges in mucins, which reduces the density and viscosity of the secretion, facilitating its expectoration. They also exhibit antioxidant, anti-inflammatory and antibacterial properties. These drugs are cheap, safe and widely available. Although mucolytic agents are not directly indicated in the GINA/GOLD guidelines in COPD or asthma, due to the lack of data from randomized clinical trials, they are widely used, particularly in asthma/COPD exacerbations. The proven effectiveness of mucolytics in reducing the risk of exacerbating COPD is particularly important. One of the mucolytics – dornase alfa – is also a key element of cystic fibrosis therapy.