Aggregation of classical fluorophores always quenches their light emission, which is notoriously known as aggregation-caused quenching (ACQ). The ACQ effect prevents many fluorophores from finding aggregation-state applications. In contrast, a group of fluorophores is weakly luminescent or even nonluminescent in isolated state but highly emissive in aggregate state. Aggregation-induced emission (AIE) was coined for this novel phenomenon. Because of their unique advantages, more and more new AIE systems with emission colors covering the entire visible spectral region were developed by numerous research groups. Their applications as solid-state emitters and chemo/bio-sensors were explored widely and deeply. Deciphering the working principle of the AIE phenomenon is of great value in terms of helping gain new photophysical insights and guide further efforts in the development of new AIE materials with high luminescence efficiencies. However, whilst the mature theories to explain the ACQ effect had been written into textbooks, the "abnormal" AIE phenomenon still poses a challenge to our current understanding of solid-state luminescence. In this review, we summarize the accessible mechanisms for the AIE phenomenon, such as restricted intramolecular rotation (RIR), intramolecular coplanarization, inhibition of intramolecular photochemical or photophysical process, relatively loose molecular packing, J-aggregate formation, and special excimer formation. Particularly, we emphasize on the description of RIR mechanism, which is the most universal and best studied one among the proposed mechanisms. In addition, some new AIE systems based on these mechanisms are introduced briefly.
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