The first-order laminar shear layer between two parallel streams has been calculated exactly for compressible and incompressible two-dimensional flow. Turbulent flow has been included by using Prandtl's hypothesis for the eddy viscosity. Results are presented for a wide range of the appropriate parameters. The problem is solved by writing the momentum and energy equations as a pair of coupled integral equations in Crocco variables; these are solved by the method of successive approximation after using a simple transformation to weaken the singularity at the outer edges of the boundary layer. This approach yields uniquely the shear stress and temperature as functions of the tangential velocity u. The so-called third boundary condition, derived by Ting (1959), is then readily satisfied in evaluating the transverse component of velocity v by quadrature. Experimental results are presented in the turbulent incompressible case. Good agreement with the exact theoretical results is obtained when one stream is much faster than the other, but this falls off as the speeds of the streams tend to equalize.