Pathogenic bacteria adhere to and colonize mucosal surfaces of the susceptible host in a highly selective manner. After the organisms penetrate the nonspecific mechanical and cleansing forces, ligands (or adhesins) on the surface of the bacteria interact in a lock-and-key fashion with complementary receptors on mucosal surfaces of the host. The adhesins are usually composed of proteins in the form of fimbriae or fibrillae and the receptors of glycolipids or glycoproteins. At the epithelial cell surfaces, two classic examples of bacterial adherence are the lipoteichoic acid-mediated attachments of group A streptococcal and the type 1 fimbriae-mediated attachment of Escherichia coli. In group A streptococci, the adhesin, lipoteichoic acid (LTA), is anchored to a protein(s) on the surface of the bacterial cells and interacts through its lipid moiety with fibronectin molecules deposited on and bound to the epithelial cells. In type 1 fimbriated E. coli, a minor 29-kDa protein located at the tip of the fimbriae interacts with D-mannose residues of glycoprotein receptors on host cells. Similar adhesin-receptor interactions have now been described for a number of pathogenic microbial agents, and undoubtedly play a central role in the early steps of the infectious process.