Carbon Dots (CDs) are a new kind of fluorescent nanomaterials developing fast in recent years. There are many articles on preparing CDs by different methods. However, summarizing and comparing these different synthesis routes, further more studying on the reason why their applications are limited in vitro was seldomreported. Herein, the property, synthesis, modification and application of CDs are comprehensively reviewed. This review focuses on CDsÂ unique properties such as low toxicity, stable fluorescence intensity and comparing the existing synthesis methods including the topdown and the bottom-up, the applications in bioimaging and analysis are also stated.