Semiconductor nanocrystals (dots, rods, wires, etc.) exhibit a wide range of electrical and optical properties that differ from those of the corresponding bulk materials. These properties depend on both nanocrystal size and shape. Compared with nanodots, nanorods have an additional degree of freedom, the length or aspect ratio, and reduced symmetry, which leads to anisotropic properties. In this paper, we report the Au nanoparticle-catalyzed colloidal synthesis of monodisperse CdS nanorods. Based on systematic high resolution transmission electron microscopy studies, we propose a growth mechanism for these nanorods.