One of the most sensitive detection methods available is chemiluminescence. Chemiluminescent substrates undergo reactions, where the product is in an excited state, usually the first triplet (T1) state (see Fig.7.1). By internal systems crossing (spin-reversal, slow!) it is converted to the first singlet (S1) state, and reaches the ground state (S0) by light emission. Even though the quantum efficiency of these reactions rarely exceed 1%, detection in the amol range is possible with photomultipliers or CCD-detectors.