Publisher Summary Every detector specification includes a picture and/or physical description of the part, including dimensions and construction. Detectors are described by certain standard figures of merit. It is important to consider the manufacturer's context for all values. A detector designed for a specific application may not be appropriate for a different application, even though the specification seems appropriate. This chapter provides an overview of detectors for fiber optics. Quantum efficiency (QE) is the ratio of the number of electron-hole pairs collected at the terminals to the number of photons in the incident light. It depends on the material from which the detector is made and is determined primarily by reflectivity, absorption coefficient, and carrier diffusion length. Photodiodes can be operated either with or without a bias voltage.