Principles of Optical Interferometry is an edited book consisting of 19 contemporaneous open-access articles featuring different aspects of optical interferometry, from heterodyne to speckle interferometry. It includes methodologies of interferometric fringe analysis, as well the applications of Fabry-Perot, Fizeau, and Mach-Zehnder Interferometers. It introduces to a class of recent interferometer designs such as the Point Diffraction Interferometer, Radial Shearing Interferometer, Fiber-Optic Ring Resonator Interferometer and a Neural Network-Based Laser Interferometer. Reading this book requires some knowledge in calculus, differential equations, electromagnetism, and general topics in interferometry. This book is intended to reach an academic audience ranging from Science undergraduate students to experienced researchers.