Nonlinear aspects of wave propagation in a random medium are considered usi
ng a systematic perturbation method. For the linear problem, this procedure
also yields a treatment that is mathematically rigorous while giving physi
cally correct results. For the special case of the propagation of a beam of
narrow frequency band of waves in a turbulent medium, the parabolic equati
on approximation is adopted for the mutual coherence function (MCF) and a F
okker-Planck equation formulation as well as a ponderomotive-force-driven m
odel are developed. (C) 2000 Elsevier Science B.V. All rights reserved.