CQDs are a new class of carbon material with ultrafine sizes and readily tunable optical properties, which make them extremely intriguing. CQDs have gained widespread attention due to their potential and versatility, and they can be applied in many different fields. One of their many applications is photocatalysis, which has garnered incessant research interest in recent years. State-of-the-art technology utilizes sustainable solar energy, which is both clean and virtually inexhaustible. To date, the photocatalytic performance of CQDs in their raw form is still far from ideal. Nevertheless, they can be substantially enhanced through several modification techniques. In this review, strategies to improve the photocatalytic performance of CQDs, such as size-tuning, surface passivation and functionalization, and elemental doping, are extensively discussed. The review also covers the latest advances in the use of CQDs in photocatalysis to address both environmental and energy-related issues. Particular emphasis is placed on the formation of 0D/2D heterojunction nanocomposites with several 2D materials, such as graphene, graphitic carbon nitride, metal oxides and metallates, metal oxyhalides, transition metal oxides and chalcogenides. The hybridization routes to binary nanocomposites, and their photocatalytic application to carbon dioxide reduction, hydrogen production, and dye and pollutant degradation, are thoroughly reviewed in this paper.