Infrequent reports of matting and tangling of hair with uncertain etiology have been published. This paper ascribes this phenomenon to "felting," a physical compaction common to most animal fibers and derived from the unique morphological, frictional, and mechanical properties of such fibers. A laboratory method for felting hair is described. Felting is experimentally enhanced by increased duration of mechanical action, by treatment with bleaching and waving solutions, and with fine or dense hair. Cosmetic cationic lubricants decrease the matting tendency while variations in shampoo composition are without effect in those examined.