Carbon dots (CDs) are an emerging carbon nanosized species with high photoluminescence quantum yields. CDs were accidentally discovered in 2004 during the isolation and purification of single-walled carbon nanotubes synthesized by arc-discharge methods. Since their discovery, various synthesis methods employing various precursors have been developed, that fall into two broad classes: top-down and bottom-up approaches. Also, CDs based composite materials have been studied widely for applications in many fields. CDs are typically quasi-spherical nanoparticles with an amorphous to nanocrystalline structure, containing sp2/sp3 carbon, oxygen/nitrogen-based groups, and superficial chemical groups. Carbon dots have attracted a broad interest in several fields due to their unique combination of optical properties, exceptional biocompatibility, minimal toxicity, great aqueous solubility, cost-effectiveness, simple synthesis, easy functionalization.