Silicon nanoparticles and carbon dots have been widely applied in solar cells, optoelectronic devices, fluorescence imaging, drug delivery, chemical detection and photocatalysis. Many strategies have been developed and significant progress has been made in the synthesis of silicon nanoparticles and carbon dots. Reducing agents are commonly applied in the synthesis of silicon nanoparticles and carbon dots. In this chapter, various types of reducing agents for the synthesis of silicon nanoparticles have been introduced. Their advantages and disadvantages were commented on. The use of reductants and their role in carbon dot synthesis have also been reviewed. The progress of silicon nanoparticles and carbon dots as reducing agents has been further presented.