Optical microresonators can tightly confine light for a long time, dramatically enhancing the photon-photon interactions and playing an increasingly important role in modern nonlinear and quantum optics. This is particularly true for whispering gallery and similar resonators based on total internal reflection. This book adopts a focused yet adaptable approach: exploring this special class of resonators in detail, it develops concepts and methods readily transferable to other resonators enabling readers to build a versatile theoretical toolkit without sacrificing depth. Both classical and quantum descriptions are developed, offering a systematic discussion of second- and third-order nonlinear optical processes in resonators. Quantum optical phenomena such as squeezing, entanglement, and coupling to non-optical quantum systems are treated in the same framework, reflecting the intrinsic link between optical nonlinearity and quantum state generation.Written for graduate students and research scientists, the book assumes familiarity with classical electrodynamics and basic quantum physics, while remaining largely self-contained. By combining physical insight, analytical modeling, and practical considerations, it fills a notable gap in the literature and serves as a comprehensive resource on extreme nonlinear and quantum optics in microresonators.