We review updated understandings of the electronic and atomic structures, relaxation dynamics and spin-multiplicity of self-trapped excitons (STEs) in alkali halide crystals, on the bases of recent experimental results of the STE luminescence. It is pointed out that seemingly complicated diversity in the features of STE luminescence and F-H pair formation can be explained from an unified viewpoint. That is, depending upon the kinds of crystals there can arise the ''adiabatic instability'', which results in multiple local minima on the adiabatic potential energy surfaces (APES's) for the singlet and triplet STE states of the lowest orbital energy. A simple phenomenological model is proposed, which indicates that the electron-hole spin interaction plays an essential role in correlation with the electron-lattice and hole-lattice interactions. The shape of the APES's, the configuration dependence of the electron-hole exchange energy, relaxation dynamics and the structure of STEs are discussed in relation to the bi-stability of parity-broken and -unbroken STEs.
