Mussels owe their sessile way of life in the turbulent intertidal zone to adaptive adjustments in the process and biochemistry of permanent attachment. These have understandably attracted scientific interest given that the attachment is rapid, versatile, tough and not subverted by the presence of water. The adhesive pads of mussel byssus contain at least six different proteins all of which possess the peculiar amino acid 3, 4-dihydroxyphenylalanine (DOPA) at concentrations ranging from 0.1 to 30 mol %. Studies of protein distribution in the plaque indicate that proteins with the highest levels of DOPA, such as mefp-3 (20 mol %) and mefp-5 (30 mol %), appear to predominate at or near the interface between the plaque and substratum. Although the presence of DOPA in proteins has traditionally been associated with cross-linking via chelate-mediated or covalent coupling, recent experiments with natural and synthetic DOPA-containing polypeptides suggest that cross-link formation is not the only fate for DOPA. Intact DOPA, particularly near the interface, may be essential for good chemisorption to polar surfaces. Uniformly high DOPA oxidation to cross-links leads to interfacial failure but high cohesive strength, while low DOPA oxidation results in better adhesion at the expense of cohesion. Defining the adaptations involved in balancing these two extremes is crucial to understanding marine adhesion.