We present some time-resolved spectroscopic studies for selected Aggregation-induced emission (AIE) systems to gain insight on their working mechanisms for the AIE processes in them.[1-3] First, we studied tetraphenylethylene (TPE)-based derivatives with varying structural rigidities and AIE characteristics were examined with. ultrafast time-resolved spectroscopy and computational studies and we discerned a direct correlation between the state-dependent coupling motions and inhibited fluorescence and confirmed the existence of photocyclized intermediates in them. This work found that the predominant non-radiative relaxation dynamics, i.e. formation of intermediate or rotation around the elongated C]C bond, induces the AIE effect, which is strongly structure-dependent but not related to structural rigidity and indicates AIE is a photophysical phenomenon correlated closely with the excited-state intramolecular motions. We next examined the nonaromatic annulene derivative of cyclooctatetrathiophene which displays a typical AIE effect in spite of its rotor-free structure.[2] This intriguing mechanism was studied with photoluminescence spectra, time-resolved absorption spectra, theoretical calculations, circular dichroism as well as by pressure-dependent fluorescent spectra etc., that indicate that the aromaticity reversal from ground state to the excited state serves to be the cause for inducing the excited-state intramolecular vibration, leading to the AIE effect in these systems. We then studied bifunctional AIEgens developed to directly deliver and visualize bioactive species together and discovered a novel photoactivatable nitric oxide (NO) donor with a built-in AIE fluorophore, that can rapidly release NO and AIEgen upon light irradiation. The ultrafast NO generation occurred in 2.1 ps, with a huge fluorescence turn-on AIEgen probe (300,000-fold intensity enhanced). Using this NO donor, real-time cell images of NO-releasing through the emission of AIEgen were successfully achieved through direct excitation of the donor in cells in situ.