A review is presented of external magnetic field effects on the fluorescence of isolated molecules which exhibit the intermediate level structure composed of singlet-triplet mixed states. Fluorescence intensity (quantum yield), decay profile (lifetime), and polarization are well affected by an external magnetic field, even when fluorescence is considered to be emitted from a diamagnetic singlet state. Effects of molecular rotation, molecular vibration, methyl internal rotation, and deuterium substitution are observed in magnetic field effects on fluorescence, and these effects are interpreted in terms of the level density of the tripler states coupled to a singlet state. It is also shown how intramolecular vibrational energy redistribution and dissociation via triplet states are related to the field dependence of fluorescence.