Fourier's Theorem is not only one of the most beautiful results of modern analysis, but it can also be regarded as an indispensable tool in addressing nearly every profound question of modern physics. Lord Kelvin, 1867. This book is a textbook that aims to familiarize the reader with the fundamental concepts of Fourier optics and to introduce them to this field to the extent that they can understand original works and independently engage with the concepts of Fourier optics. In optics, the Fourier transformation plays a threefold role. On one hand, it describes the Fraunhofer diffraction pattern of any (two-dimensional) object. Therefore, in optics, the Fourier transform of an object can be made experimentally visible (more precisely, its magnitude squared), thus providing an intuitive access to its physical significance, namely harmonic analysis.