We report on molecular-dynamics simulations of Lennard-Jones liquids sheared between two solid walls. The velocity fields, flow boundary conditions, and fluid structure were studied for a variety of wall and fluid properties. A broad spectrum of boundary conditions was observed including slip, no-slip, and locking. We show that the degree of slip is directly related to the amount of structure induced in the fluid by the periodic potential from the solid walls. For weak wall-fluid interactions there is little ordering and slip was observed. At large interactions, substantial epitaxial ordering was induced and the first one or two fluid layers became locked to the wall. This epitaxial ordering was enhanced when the wall and fluid densities were equal. For unequal densities, high-order commensurate structures formed in the first fluid layer creating slip within the fluid.