Optical Path Theory focuses on the potential to find analytical close form solutions using optical path theory to solve and design adaptive optics systems. With a thorough introduction to the subject, the first chapters of the book explore the three main axioms implemented in optical path theory: The Fermat Principle, Snells law, and the Malus-Duplin theorem. After the main axioms are presented, the book moves into exploring optical systems in the plane. The refractive mirror, refractive surface, lens, and the transfer function lens are discussed. Step-by-step equations are provided with each of these various systems to plot their waves, putting emphasis on the mathematical derivations and showing a variety of examples.