High frequency asymptotic methods lead to simplification of wave propagation problems because of a principle of localization" the high frequency behavior of solutions at a given point depends only upon local properties of the medium, boundaries and ray trajectories. This idea leads at once to the representation of solutions of complicated problems in terms of solutions of simpler, canonical problems. This idea has been pursued vigorously in recent years, and the following is an account of some of the work along these lines.In order to illustrate the basic ideas, we consider the asymptotic behavior of the Bessel function for large order and argument, i.e., we consider Jk(kr) for large k. The results may be interpreted as (i) asymptotic expansion of an integral, (ii) asymptotic solution of an ordinary differential equation, or (iii) asymptotic solution of a partial differential equation (since e-ikOJk(kr is a solution of Au + kZu 0). Here the emphasis will be on the third aspect, but there has been a continuing interaction between all three. 1. Uniform asymptotic expansion of integrals. With a suitable choice of contour, we have 1 fdk,(t,,. dr, (1.1) Jk(kr) where (1.2) (t, r) r sin t. For k >> 1 and r 1, the saddle point method is applicable (see Erd61yi [1): the saddle points i satisfy (1.3) c3t (' r) r cos 1 0. If r > 1, the saddle points are real and distinct, and we obtain the Debye approximations cos kx// 1-kcos --n/4 (1.4) Jk(kr) nkx//r2_ r If r < 1, the saddle points are complex conjugates, but the contour may be deformed to pick up only one contribution:(1.5) J(kr) 1 1/2 e -kcsh-(1/r)+k4/1 2kv/1 r 2