Solution and solid represent two fundamental states of matter. In solutions, substances dissolve into solvents, while solids form as solvents are removed. This solution-to-solid transition seems simple yet underlies many important natural phenomena, especially in molecular systems where properties in each state can vastly differ. This contrast unlocks rich molecular behaviors and provides ample opportunities for academic investigations. A typical example of this is found in organic luminescent materials. In the past two decades, organic luminescent materials have flourished with the development of modern application techniques, including flexible display, imaging, sensors, disease diagnosis and therapy, etc. One of the most significant advantages of organic luminescent materials is their excellent processability, which is manifested by the easy transition between solutions and solids. However, the luminescent properties of organic materials displayed in solutions and solids can be different, which are summarized into two complementary phenomena: aggregation-caused quenching (ACQ) and aggregation-induced emission (AIE). ACQ molecules exhibit bright emission in solutions but are quenched in aggregate states. In contrast, AIE molecules (AIEgens) are weakly emissive in solutions but become highly emissive upon aggregation. Compared to ACQ materials, AIEgens have attracted increasing attention because of their robust luminescence performance in the aggregate/solid state. This has made research on AIEgens a thriving research area. Significant progress has been made in understanding the working mechanism of AIE over the past two decades. Several theories have been proposed, among which restricted intramolecular motion (RIM) is the most popular. According to RIM, the excited molecules can readily dissipate their energy through nonradiative pathways at the molecular level due to active molecular motions. However, at aggregate levels, the motion of AIEgens is restricted, decreasing the nonradiative relaxation and consequently facilitating the radiative relaxation of AIEgens upon excitation. The RIM mechanism offers invaluable insights for tailoring AIEgens at the molecular level, which paves the foundation of AIE. Inspired by AIE, many new concepts have been discovered and proposed, such as crystallization-induced emission (CIE), cluster-triggered emission (CTE), vibration-induced emission (VIE), etc. These fantastic research outcomes lead to a conclusion that the properties of molecules in solutions and aggregated states can be vastly different, not just from the perspective of photophysics, but from the basic understanding of physical chemistry. In this context, our group proposed a new research paradigm of "aggregate science" and summarized four effects: antagonism, synergism, emergence, and divergence. The four effects cover the basic principles of aggregate science, providing general guidance for researchers and stimulating new ideas from a broad readership.