Nanosized fluorescent carbon particles, carbon dots (CDs), are a kind of fluorescent material that has many applications in recent years. They have biological application like delivery of therapeutic payloads for cancer therapy mainly due to their biocompatibility and unique optical properties. Fluorescent carbon dots overcome the short comings of high toxicity of traditional nano materials. Moreover, the preparation procedure of fluorescent carbon dots is simple and easy. Therefore, fluorescent carbon dots have great potential applied in photocatalysis, biochemical sensing, bioimaging, drug delivery and other related areas. Many functional groups or passivation agents are used to cover the surface of the CDs outside the carbon core, to make CDs with high quantum yield (QY), chemical stability and good water solubility. CDs can be easily conjugated with target molecules to expand their functionality. These traits make them an ideal alternative to semiconductor quantum dots such as CdTe and CdSe. Amoung multicoloured fluorescent carbon dots, green CDs(GCDs) shows high potential in biological labelling and bioimaging.