Gradient Index Optics deals with the application of gradients in optical systems of classical types: gradient index lenses. The emphasis is on the theory and practice related to gradient index lenses. Only isotropic media are considered since they are the ones for which the refractive index at each point is independent of direction. Comprised of 12 chapters, this book begins with a historical background on the use of gradients in astronomy and developments in gradient index lenses, along with the underlying basic theory. The discussion then turns to spherical gradients, paying particular attention to rays, Maxwell's fisheye lens, the Luneburg lens, and astronomical refraction. Subsequent chapters focus on the ray trace in a spherical gradient; axial gradients and their use as an anti-reflection coating; radial gradients and ray tracing in a radial gradient; and fundamentals of aberration theory. The wood lens and ray trace in a general medium are also considered, together with methods for fabrication of gradient elements and measurement of index gradients using an approximate method and interferometric methods. This monograph will be of interest to physicists.