I. Mucolytic Therapy Airway mucus is a two compartment liquid consisting of both an aqueous layer that supports the epithelial cilia and a gel layer that mainly contains glycoproteins (mucins) (1). The term “mucus” is also used for secretions produced in respiratory diseases such as cystic fibrosis (CF), although this term may actually be inappropriate as the composition of airway secretions in CF differs greatly from normal physiological content. The goal of mucolytic therapy is to facilitate physiological clearance by optimizing the viscoelasticity of mucus. Optimizing viscoelasticity does not equal maximal reduction of both viscosity and elasticity: therapy that lowers mucus viscosity beyond the physiological state has the potential to negatively affect its transportability, as secretions with viscosity close to that of water cannot be cleared effectively by either mucociliary or cough clearance (2). Overall, mucociliary transportability of sputum can be improved by reducing sputum viscosity, given that elasticity is preserved, while cough clearability of secretions requires higher viscosity.