Various organisms are known to attach themselves to a wide range of foreign materials in water as one of their essential physiological functions. To accomplish this, these organisms have acquired their specialized molecular systems in the process of their evolution. The molecular systems of sessile organisms are excellently designed for the purpose of underwater adhesion from the macroscopic scale to the molecular level. This review focuses on the unique sessile crustacean, the barnacle, in which a molecular system called cement was found for the underwater adhesion, which is completely different from the molecular system found in the mussel and tubeworm. The components, properties, and unique functions of the cement proteins from barnacles in comparison with those of the mussel and tubeworm are discussed.