The cloning of mucin cDNAs brought about by the application of molecular biology and molecular analyses constitutes a major step in understanding mucin structure and function. Here two classes of mucins are described: epithelium-associated and endothelium/leukocyte-associated mucins, which have thus far been described separately in the literature. The epithelial mucins are generally believed to play a role in cytoprotection. The endothelial and leukocyte class of mucins are adhesion molecules involved in lymphocyte homing and lymphocyte activation or are part of the adhesion cascade that plays a role in the initiation of inflammation. Mucins in general contain many threonine and serine residues, which are extensively O-glycosylated. Due to this profound glycosylation, mucins have a filamentous conformation. By virtue of their extended filamentous, and often negatively charged, structure, mucins can act as a barrier protecting the cell. However, when an opposing cell has specific receptors for mucins, adhesion can override the barrier function. Therefore, mucins may be powerful two-edged swords: they are both protective and adhesive.