With his first experiments, in 1913, Georges Sagnac laid the groundwork for a new category of inertial sensors, capable of measuring rotation rates by means of light. Over one century of experiments and development attempts, pushed by the increasing interest of the Inertial Navigation community in a reliable and completely strap-down class of sensors, the optical gyros gained a dominant position in a wide range of applications and grades, from tactical to high end navigation. The first uses of Sagnac effect sensors, that subsequently gained wide usage was represented by the Ring Laser Gyro (RLG). Later on, by taking advantage of the advancements in optical fiber technology, pushed by the fast growth of the Telecom market, the efforts in the development of Fiber Optic Gyroscopes (FOGs) led these highly versatile and easily scalable sensors to overcome the RLG. Thus, FOGs have recently dominated most of the rotation rate sensor technologies, especially in high performance navigation grade applications. This result by the FOG was achieved thanks primarily to its flexibility and highly advantageous, difficult to compete with, size to performance ratio. The purpose of this tutorial is to provide a basic overview of FOG technology, from the working principles to the most common implementation techniques, going through the description of the building blocks of the sensor and addressing the main design challenges and the physical effects behind them. Finally, we will discuss some of the advantages of FOGs compared to other gyro technologies.