The Parabolic Equation (PE) has applications in many different scientific fields such as electromagnetics, optics theory, quantum mechanics, plasma physics, seismology, underwater acoustics, etc. This presentation centers on a discussion of the parabolic equation approximation as applied to underwater acoustic wave propagation—past, present, and future directions. A review will be given of past contributions. Recent developments are highlighted. Looking ahead we discuss what the parabolic equation method can do in order to stimulate future research and development, as well as applications. Intensive computations with respect to the parabolic equation implementation will also be discussed.