Colloidal gold probes have become widely used for immunocytochemical staining at the electron microscopic level. Gold sols are producing by boiling a solution of tetrachloroauric acid with a reducing agent. The type of reducing agent and the concentration of components determine the final particle size. Gold sols that have a particle size ranging from 2 to 40 nm can be made in the laboratory, depending on the type and concentration of the reducing agent. This chapter details methods for producing various sizes of gold. The methods are relatively simple and very reproducible from batch to batch.