Almost all electrochemiluminescence (ECL) analytical applications are based on coreactant ECL. Because there are numerous ECL coreactants and coreactant ECL plays a dominant role in ECL bioassays, such as immunoassays, aptasensor and DNA probe assays, coreactant ECL has broad applications in the areas of food and water testing, clinical diagnostics, environmental monitoring, biowarfare agent detection and scientific research. In this chapter, we introduce various ECL coreactants, including oxidative-reduction ECL coreactants and reductive-oxidation ECL coreactants. The ECL mechanisms and applications of coreactants are discussed in detail involving diverse ECL emitters. It provides a convenient way to know ECL coreactants.