Abstract: In this paper, the results on wave propagation in nonuniform media which were studied by Lighthill, Whitham and some others are analysed. It is found that when the phase of wave is expressed as that of near plane wave i.e. θ=k (X, T)x-ω(k(X, T)X, T)t where X, T are εx, εt respectively,ε is a small parameter, k,ω are local wave number and frequency, then we can get k/T+ω/X=Ω/X.This means that the phase does not satisfy the principle of continuity as proposed by them. With this result, it is pointed out that the wave number is always constant along the rays while the frequency is variable. This is different from that obtained by Lighthill and others. The causes of these differences are discussed in the paper.