Planar waveguides are a class of devices capable of controlling light on a chip to realize performance advantages over ordinary building blocks of free-space optics. They are increasingly becoming attractive in various systems due to their unique features of large evanescent fields, compactness, and, most importantly, their ability to be configured according to the required application. For designing and optimizing the planar waveguides, a comprehensive understanding of the behavior of the light in the waveguide needs to be achieved. This chapter overviews and discusses the propagation of light in planar waveguides.