  Recent results relating to ray dynamics in ocean acoustics are reviewed.
Attention is focussed on long-range propagation in deep ocean environments. For
this class of problems, the ray equations may be simplified by making use of a
one-way formulation in which the range variable appears as the independent
(time-like) variable. Topics discussed include integrable and non-integrable
ray systems, action-angle variables, nonlinear resonances and the KAM theorem,
ray chaos, Lyapunov exponents, predictability, nondegeneracy violation, ray
intensity statistics, semiclassical breakdown, wave chaos, and the connection
between ray chaos and mode coupling. The Hamiltonian structure of the ray
equations plays an important role in all of these topics.
