To be useful in high performance optoelectronic device applications like in OLEDs and lasers, it is essential to develop emissive ESIPT systems operating not only in a dilute solution state but also in the condensed solid-state with practically high fluorescence quantum yield. However, the fluorescence efficiency of the large Stokes' shifted K* emission is generally very low due to the presence of various non-radiative deactivation pathways of K* state, which are typical in ESIPT molecules. Recently, various approaches to suppress those deactivation pathways and thus to enhance the emission efficiency have been developed as follows.