Mucus is a viscous secretion produced by goblet cells and submucosal glands found in mucous membranes throughout the body. Chemically, mucus consists primarily of water, glycoproteins called mucins, electrolytes, immunoglobulins, and cellular debris including leukocytes. Its primary functions are lubrication of epithelial surfaces, trapping and neutralizing pathogens, and facilitating mucociliary clearance in the respiratory tract. In horses, abnormal mucus characteristics are important diagnostic indicators. Thick, yellow-to-green nasal discharge suggests bacterial upper respiratory infection, while clear or lightly cloudy nasal mucus can accompany viral infections such as equine influenza or equine herpesvirus. Excess mucus in the trachea, visible on endoscopy, is graded on a four-point scale and is associated with inflammatory airway disease and recurrent airway obstruction. Rectal examination of a colicky horse routinely assesses the character of intestinal mucus. For broader context see intestinal mucus assessment during colic evaluation and parasites that can affect intestinal mucus production.