Electrochemiluminescence or electrogenerated chemiluminescence (ECL) is a light-emission phenomenon that involves redox reactions at the electrodes, electron transfer between the electrochemically generated redox species, the formation of excited states of luminescent molecules, and the radiative electronic transition of the luminescent molecules. Light-emitting devices based on ECL, which generally consist of two electrodes and a phase containing one or several types of luminescent molecules, can be very easily fabricated. In this chapter, the principle of ECL and the main components of ECL devices (ECLDs), such as electrodes, solvents, and gels, are briefly described. Furthermore, characteristics including the device performance of several ECLDs based on organic solvents, ionic liquids, liquid luminescent molecules, and gel are introduced.