Book excerpt: A number of well-known theorems of the hydrodynamic theory of stability are interpreted in terms of the interaction of the waves having different energy signs. Attention is drawn to the plasma-hydrodynamic analogy, which is a powerful tool for physical analyses of general mechanisms of wave amplification and absorption in flows. Various wave-flow interaction problems are considered, for instance, sound generation in whistlers, wave scattering and amplification by vortices, methods of wave remote sounding, and some nonlinear dynamical and chaotic phenomena.