Carbon quantum dots (CQDs) have aroused much attentions for their excellent ROS regulating properties. However, there is still few reports systematically discuss the dual properties of CQDs for ROS-generating and ROS-scavenging. This review attempts to systematically summarize the mechanism and structure-activity relationship of CQDs on ROS-regulation, and comprehensively summarize their biomedical applications. Generally, ROS generation could be attributed to the coexistence of the excitonic and charge-carrier aspects, while scavenging of free radicals can be attributed to electron transfer or hydrogen donation ability of the sp2 carbon core, functional surface groups and other introduced reaction sites. The intrinsic ROS generating and/or scavenging properties could be tuned through doping with nonmetal/metal elements, combining with metal nanoparticles, combining with semiconductors, etc. Especially, it is interesting to note that doping with non-metal elements (such as N, P, S, Cl) or combining with gold nanoparticle could both enhance ROS generating and scavenging ability of CQDs. Thus, CQDs with appropriate design could be used as ROS-generator or -scavenger depending on ambient conditions (the presence or absence of light source) for anti-tumor, anti-bacterial, wound healing, etc. •CQD has dual functions of ROS-generation and -scavenging.•ROS is generated via type Ⅰ and Ⅱ pathways under light.•Enhancing e−-h+ separation could increase ROS generation.•ROS is cleared by carbon core, surface group, etc.•Raising e transfer or H donating increase ROS scavenging.