Following an overview of the primitive state of mechanistic studies of the formation of nanoclusters, with a focus on LaMer's classic work on the formation of sulfur sols, kinetic and mechanistic studies of the formation of our recently reported novel P2W15Nb3O629- polyoxoanion- and Bu4N+ -stabilized Irsimilar to 190-450 (hereafter, lr(0)(similar to 300)) nanoclusters are presented. The work reported consists of the full experimental and other details of the following eight major components: (i) development of an indirect-but easy, continuous, highly quantitative and thus powerful-method to monitor the formation of the Ir(0) nanoclusters via their catalytic hydrogenation activity and through the concept of pseudoelementary reaction steps; (ii) application of the appropriate kinetic equations for nucleation and autocatalysis, and then demonstration that these equations fit the observed, sigmoidal-shaped kinetic curves quantitatively with resultant rate constants k(1) and k(2); (iii) confirmation by a more direct, GLC method that the method in (i) indeed works and does so quantitatively, yielding the same k(1) and k(2) values within experimental error; (iv) collection of a wealth of previously unavailable kinetic and mechanistic data on the effects on nanocluster formation of added olefin, H-2 pressure, anionic nanocluster stabilizer ([Bu4N](9)P2W15Nb3O62 in the present case), H2O, HOAc, and temperature; (v) careful consideration and ruling out of other hypotheses, notably that particle-size rate effects alone might account for the observed sigmoidal shaped curves; and then (vi) distillation of the results into a minimalistic mechanism consisting of several pseudoelementary steps. Also presented as part of the Discussion are (vii) a concise but comprehensive review of the literature of transition metal nanocluster formation under H-2 as the reducing agent, an analysis which provides highly suggestive evidence that the new mechanism uncovered is a much more general mechanism-if not a new paradigm-for transition metal nanoclusters formed under H? (and, the data argue, probably also for related reducing agents); and (viii) a summary of the seven key predictions of this new mechanism which remain to be tested (four predictions are the expected predominance of magic-number size nanoclusters; designed control of nanocluster size via the Living-metal polymer concept; the synthesis of onion-skin structure bi-, tri-, and higher-metallic nanoclusters; and the use of face-selective capping agents as a way to block the autocatalytic surface growth and, thereby, to provide designed-shape nanoclusters). Overall, it is hoped that the results-the first new mechanism in more than 45 years for transition metal nanocluster formation-will go far toward providing a firmer mechanistic basis, and perhaps even a new paradigm, for the designed synthesis of new transition metal nanoclusters of prechosen sizes, shapes, and mono-to multimetallic compositions.