Mucolytic and related agents have been in use since prehistoric times. Although widely prescribed and used extensively in over-the-counter preparations, their efficacy and mechanisms of action remain in doubt. These agents belong to several distinct chemical classes. Mucolytic agents such as N-acetyl-cysteine are thiols with a free-sulfhydryl group. They are assumed to break disulfide bonds between gel-forming mucins and thus reduce mucus viscosity. Mucokinetic agents are thiols with a blocked sulfhydryl group. Expectorants such as guaifenesin increase mucus secretion. They may act as irritants to gastric vagal receptors, and recruit efferent parasympathetic reflexes that cause glandular exocytosis of a less viscous mucus mixture. Cough may be provoked. This combination may flush tenacious, congealed mucopurulent material from obstructed small airways and lead to a temporary improvement in dyspnea or the work of breathing. The roles of anticholinergic agents, DNase, and other drugs are also discussed with regard to their roles in reducing mucus production in rhinitis and other airway diseases.