This book explains how to understand and analyze the working principles of optical systems by means of optical theories and case studies. Part I focuses mainly on the theory of classical optics, providing an introduction to geometrical and wave optics, and some concepts of quantum and statistical optics. Part II presents case studies of three practical optical systems that comprise important and commonly used optical elements: confocal microscopes, online co-phasing optical systems for segmented mirrors, and adaptive optics systems. With the theoretical background gained in Part I, readers can apply their understanding of the optical systems presented in Part II to the conception of their own novel optical systems. The book can be used as a text or reference guide for students majoring in optics or physics. It can also be used as a reference for any scientist, engineer, or researcher whose work involves optical systems.