We describe some basic procedures for studying the properties of mucus. These techniques can be applied in both clinical and physiological studies to improve the understanding of the mechanisms related to epithelial defense in health and disease. Mucus collection--A major difficulty is the lack of simple and noninvasive methods for collecting normal mucus in sufficient quantity for later analysis. Physical properties of mucus--A. Rheology: Mucus exhibits both solid and liquid properties and the important factor governing the actual behavior is time. The magnetic microrheometer provides an elegant method for measuring rheological properties of microsamples. B. Adhesivity: It characterizes the forces of attraction between an adherent surface and an adhesive system and can be calculated by measuring the contact angle between a mucus drop and a surface. Mucus Transport--A. Mucus transport by cilia: Mucus is primarily cleared by the continuous ciliary beating, which can be studied using techniques such as the frog palate preparation as well as direct measurement, i.e., in situ mucus clearance. B. Cough clearance: It is essential for elimination of secretions in diseases leading to hypersecretory states. The cough machine simulates the flow-time profile of human coughing. Transepithelial potential difference--A potential difference exists between the epithelial surface and the submucosa and is the net result of the activity of the ion-transport system of the pulmonary epithelium. The potential can be measured using appropriate microelectrodes. Quantitative morphology--Methods may be used to characterize the epithelial surface condition that continuously changes during aggressive conditions.