The structural and thermodynamic properties of tethered polymer layers formed by spreading diblock copolymers at it solid surface or at a fluid-fluid interface are studied using a molecular mean-field theory. The role of the anchoring block in determining the properties of the tethered polymer layer is studied in detail. It is found that both the anchoring and the tethered blocks are very important in determining the phase behavior of the polymer layer. The structures of the coexisting phases, the phase boundaries and the stability of the layer are found to depend on the ratio of molecular weight between the two blocks, the polymer-interface (surface) interactions and the strength of the interactions between the two blocks. The different phase transitions found are related to mental observations. The properties of the polymer layers at coexistence reflect the block that is the dominant driving force for phase separation. The ability of the tethered polymer layers, under different conditions, to control protein adsorption to surfaces is also studied. It is found that the most important factors determining the ability of a polymer layer to reduce the equilibrium amount of proteins of adsorbed to a surface are the surface coverage of polymer and the surface-polymer interactions. The polymer chain length plays only a secondary role. For the kinetic control, however, it is found that the potential of mean-force, and thus the early stages of adsorption, depends strongly on polymer molecular weight. Further, it is found that the molecular factors determining the ability of the tethered polymer layer to reduce the equilibrium :df protein adsorption are different than those that the kinetic behavior. Comparisons with experimental observations are presented. The predictions of the theory are in very good agreement with the measured adsorption isotherms. Guidelines for building optimal surface protection for protein adsorption, both kinetic and thermodynamic, are discussed.