A revised view of Brust-Schiffrin metal nanoparticle syntheses is presented here. Precursor species of these reactions are identified and quantified for Au, Ag, and Cu systems. Contrary to the assumptions of previous reports, tetraalkylammonium metal complexes are shown to be precursors of the two-phase reactions, whereas M(I) thiolates are shown to be precursors of the one-phase reactions. A new scheme is outlined for the two-phase synthesis, and the implications of this scheme are discussed. A new synthetic strategy employing well-defined precursors is also introduced. Finally, M(I) thiolate formation, and its impact on nanoparticle synthesis, is discussed. It is expected that the results presented here will lead to modifications in the manner in which these important syntheses are conducted.