Mucus, a viscous colloid gel, is an important element of cell defense developed in the course of evolution by live systems. Mucins, the main components of mucus, are glycoproteins characterized by specific structure and functions. Two subfamilies of the large mucin superfamily have been identified: secreted mucin glycoproteins and membrane-bound mucins. The secreted mucins are further subdivided into two groups: insoluble gel-forming mucins and soluble mucins. All gel-forming mucins share several features, such as specific domain structures, glycosylation patterns and biosynthetic pathways that differ from those of the membrane-bound mucins. Several classifications of the mucin glycoproteins have been proposed, but no one is universal. Further studies of the mucins are needed for development of an appropriate classification system.