One of the most profound properties of light is its wave nature and the ability to diffract. This refers to the deviation of light from a from straight line during propagation and is apparent when light passes through sharp edges. This immediately makes us wonder whether light is more than a geometric ray and prompts us to consider that it is also a wave just like water or sound waves. So, the simplest yet complete way to understand the diffraction nature of light and its properties is to consider it as a wave. Accordingly, we discuss the wave theory of light based on a set of postulates comprising the wave equation and light wave propagation in different mediums. Subsequently, the wave theory is used to understand the diffraction of light. Specifically, we will focus our discussion on the diffraction of light by an objective lens, which is central to super-resolution microscopy. In addition, the diffraction property will help us determine the minimum solvable distance i.e., resolution limit (Abbe/Rayleigh limit). This will identify the factors affecting the resolution limit of an optical microscope. The chapter will set the stage for numerous microscopy systems capable of super-resolution.