There is growing physical and mathematical interest in the hydrodynamics of
dissipationless/dispersive media. Since G.~B.~Whitham's seminal publication
fifty years ago that ushered in the mathematical study of dispersive
hydrodynamics, there has been a significant body of work in this area. However,
there has been no comprehensive survey of the field of dispersive
hydrodynamics. Utilizing Whitham's averaging theory as the primary mathematical
tool, we review the rich mathematical developments over the past fifty years
with an emphasis on physical applications. The fundamental, large scale,
coherent excitation in dispersive hydrodynamic systems is an expanding,
oscillatory dispersive shock wave or DSW. Both the macroscopic and microscopic
properties of DSWs are analyzed in detail within the context of the universal,
integrable, and foundational models for uni-directional (Korteweg-de Vries
equation) and bi-directional (Nonlinear Schr\"{o}dinger equation) dispersive
hydrodynamics. A DSW fitting procedure that does not rely upon integrable
structure yet reveals important macroscopic DSW properties is described. DSW
theory is then applied to a number of physical applications: superfluids,
nonlinear optics, geophysics, and fluid dynamics. Finally, we survey some of
the more recent developments including non-classical DSWs, DSW interactions,
DSWs in perturbed and inhomogeneous environments, and two-dimensional, oblique
DSWs.Comment: review article, 68 pages, 52 figure