This critical review by Yuning Hong, Jacky W. Y. Lam, and Ben Zhong Tang provides an extensive overview of the aggregation-induced emission (AIE) phenomenon, which has gained significant attention since its inception in 2001. The review highlights recent advancements in AIE research, discussing typical examples of AIE systems and their structural-property relationships. Through mechanistic analysis of photophysical processes, the authors develop strategies for designing new AIE luminogens. The review also explores technological applications, particularly in optoelectronics and biology, to illustrate how the AIE effect can be utilized for high-tech innovations. The introduction explains the historical context of luminescence studies, emphasizing the contrast between dilute and concentrated solutions. It discusses the "aggregation-caused quenching" (ACQ) effect, where light emission is quenched due to the formation of aggregates in concentrated solutions. This effect is detrimental to practical applications, such as sensors and OLEDs, due to its negative impact on luminescence efficiency. The review then delves into the discovery of AIE, where the authors found that certain luminogens, like hexaphenylsilole (HPS), exhibit enhanced emission when aggregated in concentrated solutions or solid states. This phenomenon, termed "aggregation-induced emission" (AIE), is the opposite of ACQ and offers new opportunities for technological advancements. The review covers various classes of AIE luminogens, including hydrocarbon, heterocyclic, supramolecular, polymeric, and organometallic systems. It discusses the structural features that contribute to AIE, such as planarity, rotatability, and intermolecular interactions. The authors provide detailed examples and mechanisms for each class, highlighting the importance of intramolecular rotations (RIR) in activating the AIE effect. The review also explores the use of heteroatoms and hydrogen bonding to enhance the emission colors and stability of AIE luminogens. It discusses the development of AIE systems with red-shifted emissions and the role of photoresponsive molecules like azobenzene. Finally, the review examines the application of AIE in polymeric systems, including the development of AEE (aggregation-enhanced emission) polymers and hyperbranched polymers. It also introduces novel luminogens without traditional chromophores, such as tetraphenylethane (s-TPE) and glycodynamers, which exhibit crystallization-induced emission (CIE). The mechanical discussion section aims to clarify the underlying mechanisms of AIE, focusing on the role of planarity, rotatability, and intermolecular interactions. It provides insights into how these structural aspects influence the light-emitting behaviors of AIE luminogens and offers guidance for designing new AIE systems. Overall, the review serves as a comprehensive guide to the current state of AIE research, highlighting its potential for technological advancements and biological applications.This critical review by Yuning Hong, Jacky W. Y. Lam, and Ben Zhong Tang provides an extensive overview of the aggregation-induced emission (AIE) phenomenon, which has gained significant attention since its inception in 2001. The review highlights recent advancements in AIE research, discussing typical examples of AIE systems and their structural-property relationships. Through mechanistic analysis of photophysical processes, the authors develop strategies for designing new AIE luminogens. The review also explores technological applications, particularly in optoelectronics and biology, to illustrate how the AIE effect can be utilized for high-tech innovations. The introduction explains the historical context of luminescence studies, emphasizing the contrast between dilute and concentrated solutions. It discusses the "aggregation-caused quenching" (ACQ) effect, where light emission is quenched due to the formation of aggregates in concentrated solutions. This effect is detrimental to practical applications, such as sensors and OLEDs, due to its negative impact on luminescence efficiency. The review then delves into the discovery of AIE, where the authors found that certain luminogens, like hexaphenylsilole (HPS), exhibit enhanced emission when aggregated in concentrated solutions or solid states. This phenomenon, termed "aggregation-induced emission" (AIE), is the opposite of ACQ and offers new opportunities for technological advancements. The review covers various classes of AIE luminogens, including hydrocarbon, heterocyclic, supramolecular, polymeric, and organometallic systems. It discusses the structural features that contribute to AIE, such as planarity, rotatability, and intermolecular interactions. The authors provide detailed examples and mechanisms for each class, highlighting the importance of intramolecular rotations (RIR) in activating the AIE effect. The review also explores the use of heteroatoms and hydrogen bonding to enhance the emission colors and stability of AIE luminogens. It discusses the development of AIE systems with red-shifted emissions and the role of photoresponsive molecules like azobenzene. Finally, the review examines the application of AIE in polymeric systems, including the development of AEE (aggregation-enhanced emission) polymers and hyperbranched polymers. It also introduces novel luminogens without traditional chromophores, such as tetraphenylethane (s-TPE) and glycodynamers, which exhibit crystallization-induced emission (CIE). The mechanical discussion section aims to clarify the underlying mechanisms of AIE, focusing on the role of planarity, rotatability, and intermolecular interactions. It provides insights into how these structural aspects influence the light-emitting behaviors of AIE luminogens and offers guidance for designing new AIE systems. Overall, the review serves as a comprehensive guide to the current state of AIE research, highlighting its potential for technological advancements and biological applications.