As a new class of fluorescent carbon materials, graphene quantum dots (GQDs) have drawn increasing attention due to their outstanding properties and potential applications. Herein, GQDs prepared by chemical synthetic strategies, which mainly contain cutting different carbon resources and carbonization from small molecules or polymers, are discussed. Although the photoluminescence (PL) mechanism of GQDs can be explained by the size of the GQDs, surface chemical groups, and doping atoms, there is no universal agreement on the specific PL mechanism. The dominant PL mechanism of GQDs consists of the quantum confinement effect of conjugated pi-domains, the surface/edge state in GQDs, as well as the synergistic effect of these two factors. (C) 2016 Elsevier Ltd. All rights reserved.