Huygen's principle, in the form of boundary integral equations, is applied to the problem of radio wave propagation. Complex propagation is analyzed by dividing the region between transmitter and receiver into a number of zones and propagating the solution between these zones by means of integral equations with simple approximate kernels. In the limit that the regions become dense, the approach leads to a useful form of the WKB solution, and this is demonstrated through its application to the study of propagation through a disturbed ionosphere.