Fiber-optic technology has its own distinct set of measurements, based largely on a mixture of optical and electronic techniques. This chapter covers the important optical aspects of fiber-optic measurements, assuming you know something about electronics. It covers measurement units, the quantities measured, and the types of measurements performed, pointing out differences between optical and electronic measurements. The emphasis here is on basic optical concepts that you need to know when working with fiber optics. The next chapter covers fiber-optic test equipment, along with its use in troubleshooting fiber-optic systems.