When optical signals transmitted in optical fiber arrive at the destination, they have to be converted to electronic signals. Photodetectors are used for this purpose.1, 2 The converted electronic signals are then further amplified by electronic amplifiers or demodulated to reproduce the original signals from the transmitter side. If the optical signals arriving at the photodetectors are very weak, they may be amplified by an optical preamplifier before they are sent to the photodetectors. In this chapter, we will first focus on the working principle of photodetectors that convert the optical signals to electronic signals.3 Several types of photodetectors commonly used for optical communication will be introduced. Then the sources of noise associated with signal detection will be also explained. Finally, an evaluation of the signal-to-noise ratio will be discussed.