Here, a strategy for synthesizing monodisperse Ag-Au alloy nanoparticles whereby particle attributes such as morphology, composition, size, and surface chemistry may be independently controlled, varied, and customized is presented. The synthesis uses a multi-step procedure to deliver control of morphology, size, and composition in discrete and independent steps. Specifically Ag nanoparticles with the same morphology but different sizes are first prepared by the chemical reduction of Ag ions. A digestive ripening post-treatment followed by seed-mediated growth is then applied to narrow the size distribution and to vary the particle size. Monodisperse Ag-Au alloy nanoparticles are then formed by a replacement reaction with HAuCl4. Both single-crystalline truncated octahedral (TO) Ag-Au alloy nanoparticles and icosahedral multiply twinned particles can be easily prepared by this procedure. By using truncated octahedrons as the model morphology, the syntheses of nanoparticles with the same size but different compositions, of nanoparticles with the same composition but variable sizes, and of nanoparticles with different surface chemistry are demonstrated and discussed in detail. Because of the shape and size monodispersity, all of the as-synthesized Ag-Au alloy nanoparticles easily form superlattices on a solid substrate upon slow evaporation of the solvent. The packing pattern of the nanoparticles is strongly dependent on the native morphology of the nanoparticles.