Diffraction is the result of any wave (electromagnetic and/or mechanical) deviating from its normal straight path due to the presence of an obstacle or aperture of a size comparable to the wavelength of the incident wave. The sharp edges of the obstacle or aperture causes the diffraction effect. The projection of the diffracted wave, after passing around or through the obstacle or aperture, onto a screen forms a real image that is called a diffraction pattern. The features of a diffraction pattern depend on the physical geometry and kind of the obstacle. Among the simplest setups to observe such a phenomenon consists of a source of light, an obstacle with a slit, a screen, and a lens or lenses. The arrangement of these elements is based on one of two considerations: one that allows for diffraction of plane waves focused on a screen via a lens; thus, the source and the screen are effectively treated as if they were at infinite distances from each other; this is known as Fraunhofer diffraction. The other allows for diffraction of spherical waves received on a screen at a finite distance from the source; this is called Fresnel diffraction. This chapter presents a detailed discussion of Fraunhofer diffraction.