Secreted mucins are a family of polydisperse, high molecular mass and highly glycosylated glycoproteins synthesized by mucosae, or skin of amphibians and fish. Most secreted mucins are synthesized in goblet cells of epithelial surfaces or mucous cells of exocrine glands. They are probably the most complicated biological molecules ever described. Except for a low molecular mass species, the peptide part of mucin, or apomucin, is very long and can contain from 5,000 to more than 13,000 amino acids. In man, the apomucins synthesized in goblet or mucous cells are encoded by four mucin genes (MUC), organized in a cluster. Apomucins are covered by hundreds of carbohydrate chains. Their biosynthesis implicates different glycosylation processes, mostly O-glycosylation, which are responsible for about 80 % of the mucin molecular mass. During their biosynthesis, secreted mucins may dimerize and multimerize via disulfide bridges. The carbohydrate chains resulting from the O-glycosylation process are extremely diverse and contribute to the possible formation of different mucin glycoforms from a single apomucin peptide. In man, the glycosylation differs from one individual to another according to histo-blood bood groups (ABO, secretor, Lewis...). Moreover, there are large differences in the O-glycosylation processes between different animal species. Preliminary data suggest that different factors endogeneous (cytokines and / or hormones), and exogeneous (microbial), are involved in the regulation of mucin biosynthesis. Owing to their physical properties, secreted mucins have an essential role in the protection of underlying epithelia. Mice genetically deficient in one of the mucin genes (MUC2) frequently develop tumors. Finally, the wide diversity of carbohydrate chains, specific for a given species, is very important in specific recognition phenomena (interactions with microbes, fertilization...).