This book provides a clear, concise, and consistent exposition of what aberrations are, how they arise in optical imaging systems, and how they affect the quality of images formed by them. The emphasis of the book is on physical insight, problem solving, and numerical results, and the text is intended for engineers and scientists who have a need and a desire for a deeper and better understanding of aberrations and their role in optical imaging and wave propagation. Some knowledge of Gaussian optics and an appreciation for aberrations would be useful but is not required. The second edition of Aberration Theory Made Simple features an updated Cartesian sign convention, which is used in advanced books on geometrical optics and in optical design software. New topics include centroid and standard deviation of ray aberrations, spot diagrams for primary aberrations, the golden rule of optical design about relying on such diagrams, update of 2D PSFs for primary aberrations, aberration-free optical transfer function of systems with annular and Gaussian pupils, Zernike polynomials for circular, annular, and Gaussian pupils, effect of longitudinal image motion on an image, lucky imaging in ground-based astronomy, and adaptive optics.