Carbon dots (CDs) are one of the most popular carbon nanomaterials after the fullerenes, carbon nanotubes and graphene. The emergence of carbon dots has caused great interest among researchers, because CDs have excellent water solubility, good chemical inertness, hypotoxicity, good fluorescence properties, favorable biocompatibility, excellent environmental friendly, resistance to photobleaching. Moreover, CDs are easier for functional modification. In this review, we systematically describe the preparation of CDs, we hope to stimulate more researchers to research in this field, especially for their potential applications in biomarkers, biological sensing, biochemical analysis, optoelectronic devices, photocatalysis and drug carrier and so on.