Marine mussels (Mytilus) are experts at bonding to a variety of solid surfaces in a wet, saline and turbulent environment. Bonding is rapid, permanent, versatile and protein-based. In mussels, adhesive bonding takes the form of a byssus--a bundle of extracorporeal threads--each connected to living tissues of the animal at one end and secured by an adhesive plaque at the other. We have investigated the composition and formation of byssal plaques and threads with the hope of discovering technologically relevant innovations in chemistry and materials science. All proteins isolated from the byssus to date share the quality of containing the unusual amino acid, 3,4-dihydroxyphenylalanine. This residue appears to have a dual functionality with significant consequences for adsorption and cohesion. On the one hand, it forms a diverse array of weaker molecular interactions such as metal chelates, H-bonds, and pi-cations: these appear to dominate in surface behavior (adsorption). On the other hand, 3,4-dihydroxyphenylalanine and its redox couple, dopaquinone, can mediate formation of covalent cross-links among byssal proteins (cohesion). One of the challenges in making functional biomimetic versions of byssal adhesion is to understand how these two reactivities are balanced.