Fluorescent materials have gained extensive attention owing to their highly photophysical properties and potential applications in diverse areas. Aggregation-induced emission (AIE) is a new type of photophysical phenomenon discovered in 2001, in which molecules in condensed solution or solid state exhibit enhanced fluorescence emission. In this chapter, we aim to cover the developments in the exciting field of AIE over the past decade, including organic luminescent molecules and clusteroluminogens, of which clusteroluminogens are highlighted. The fluorescence mechanism is one of the most important concepts within the scientific community. Therefore, we systematically summarize the latest research achievements on the luminescence mechanism of AIE-based fluorescent systems.