The problem of predicting the characteristics of a hypersonic laminar boundary layer that interacts with the external flow field is approached using the tangent wedge formulation for the inviscid flow field and the method of similar solutions for the viscous flow. It is shown that the concept of local similarity which allows the pressure gradient parameter ft to vary in the streamwise direction leads to an explicit relation between the viscous and inviscid flows for all values of the hypersonic interaction parameter %«,. The conditions of strong and weak interaction appear as asymptotic limits of the general relations. The present theory is compared with three independent experimental investigations. In each case, the agreement is found to be excellent over the range of %oo investigated. It is shown, using asymptotic solutions to the exact boundary layer equations, that the present theory is applicable to a wide variety of viscous interaction problems. A large number of solutions to the laminar boundary layer similarity equations for a perfect gas with cross flow and surface mass transfer are given. These numerical results, when combined with the solutions of previous authors, are sufficient to describe the range of conditions 0 < 1, with high precision.