Marine mussels secrete adhesive proteins that enable these organisms to bind tenaciously to surfaces in a wet, saline, and turbulent environment. One of the unique structural features of these adhesive proteins is the presence of DOPA, an amino acid consisting of an adhesive catechol side chain. This chapter reviews the reversible and irreversible chemistries of catechol and various methods in creating catechol-functionalized polymeric materials. Finally, recent applications of these biomimetic polymers in designing tissue adhesives, antifouling coatings, delivery vehicles for therapeutic drugs and cells, and smart hydrogels and adhesives are reviewed.