This book explores recent developments in QIA and describes the application of the theory to different branches of wave physics, from plasma physics, quantum physics, and ionospheric radio wave propagation to acoustics, optics, and astrophysics. This is an up-to-the-minute exposition of the latest developments in an important new area, written by authors of outstanding reputation. A rich source of both theoretical methods and practical applications, it covers a wide range of problems of general physical significance. Until recently, there was no effective method for describing waves in weakly anisotropic inhomogeneous media. The method of quasi-isotropic approximation (QIA) of geometrical optics was developed to overcome this problem. The QIA approach bridges the gap between geometrical optics of isotropic media (Rytov method) and that of anisotropic media (Courant-Lax approach), thus providing a complete picture of the geometrical optics of inhomogeneous media.