This is a study of the three-dimensional boundary layer over the windward side of a flat delta wing in hypersonic flow at a moderate angle of attack. The interaction between the inviscid and the viscous flow regions is analyzed, and its effect is taken into account. The theory is developed for thin, flat delta wings with the shock detached from the leading edges and attached at the apex. The inviscid solution based on the Newtonian-conical flow theory of Messiter-Hida has been matched to the three-dimensional boundary-layer solution over the wing. The transverse velocity component normal to the conical ray is small, and a perturbation method leads to a system of ordinary differential equations. The solution of these equations together with the tangent-cone method is used to study the viscous-inviscid interaction. The results are applied to calculate the streamline pattern, induced pressure, skin drag, and heat transfer over the wing surface. The present theory is in general agreement with the available experimental data.