A presentation of the state of the art in a theory of oscillatory and wave phenomena in hydrodynamical flows. It uses a unified approach for waves of different physical origins. A characteristic feature of this approach is that hydrodynamical phenomena are considered in terms of physics - that is, the complement of the conventionally employed formal mathematical approach. Some physical concepts such as wave energy and momentum in a moving fluid are analyzed, taking into account induced mean flow. The physical mechanisms responsible for hydrodynamic instability of shear flows are considered...