Carbon quantum dots (CQDs) are small carbon nanoparticles (less than 10 nm) with unique properties that have found wide applications in various fields. This article reviews recent progress in the synthesis, size control, modification, optical properties, luminescent mechanism, and applications of CQDs in biomedicine, optronics, catalysis, and sensors. CQDs are synthesized through various methods, including chemical ablation, electrochemical carbonization, laser ablation, microwave irradiation, and hydrothermal/solvothermal treatment. Size control is achieved through confined pyrolysis, and surface functionalization is used to tune their properties for specific applications. CQDs exhibit strong luminescence, high solubility, and good biocompatibility, making them suitable for bioimaging, biosensors, and drug delivery. Doping with elements like nitrogen, sulfur, and phosphorus enhances their luminescent properties. CQDs can also form nanohybrids with inorganic nanoparticles, enhancing their functionality. The optical properties of CQDs are influenced by their size, surface chemistry, and the presence of defects. CQDs show promise in biomedical applications, including bioimaging, due to their low toxicity and high biocompatibility. They are also used in optoelectronics, catalysis, and sensors. The luminescent mechanism of CQDs is attributed to the recombination of electron-hole pairs in sp² carbon clusters. CQDs have potential in various applications, including cell imaging, drug delivery, and environmental monitoring. Their unique properties and versatility make them a promising material for future technological developments.Carbon quantum dots (CQDs) are small carbon nanoparticles (less than 10 nm) with unique properties that have found wide applications in various fields. This article reviews recent progress in the synthesis, size control, modification, optical properties, luminescent mechanism, and applications of CQDs in biomedicine, optronics, catalysis, and sensors. CQDs are synthesized through various methods, including chemical ablation, electrochemical carbonization, laser ablation, microwave irradiation, and hydrothermal/solvothermal treatment. Size control is achieved through confined pyrolysis, and surface functionalization is used to tune their properties for specific applications. CQDs exhibit strong luminescence, high solubility, and good biocompatibility, making them suitable for bioimaging, biosensors, and drug delivery. Doping with elements like nitrogen, sulfur, and phosphorus enhances their luminescent properties. CQDs can also form nanohybrids with inorganic nanoparticles, enhancing their functionality. The optical properties of CQDs are influenced by their size, surface chemistry, and the presence of defects. CQDs show promise in biomedical applications, including bioimaging, due to their low toxicity and high biocompatibility. They are also used in optoelectronics, catalysis, and sensors. The luminescent mechanism of CQDs is attributed to the recombination of electron-hole pairs in sp² carbon clusters. CQDs have potential in various applications, including cell imaging, drug delivery, and environmental monitoring. Their unique properties and versatility make them a promising material for future technological developments.