So far, I have concentrated on how optical fibers are used for communications. However, fiber optics also have other important uses. This chapter will show how fibers are used as sensors. Fiber sensors work in a variety of ways, sometimes just using fibers to deliver light, other times monitoring changes induced in light transmission caused by external effects. Fiber sensors can measure pressure or temperature, serve as gyroscopes to measure direction and rotation, sense acoustic waves at the bottom of the sea, and do many other tasks.