We review the effect of doping on the optical properties of luminescent colloidal carbon dots. They are considered as a hybrid material featuring both molecular and semiconductor-like characteristics, where doping plays an important role. Starting from the short overview of synthetic strategies, we consider the evolution of carbon dots from molecular precursors to fluorescent nanoparticles, and the relevant structural properties of carbon dots. Choice of the reactant materials, dopant atoms and reaction parameters provide carbon dots with varying optical properties. High chemical stability, bright luminescence and customizable surface functionalization of carbon dots open their use in a broad range of applications, which are exemplary presented at the end of this review.