A review of the literature of kinetic and mechanistic Studies of transition-metal nanocluster nucleation and growth is presented; the focus is on nucleation processes. A brief survey of nucleation theory is given first, with an emphasis on classical nucleation theory, as this is the logical starting point of transition-metal nanocluster nucleation and growth studies. The main experimental methods for following nanocluster formation are examined next-dynamic light scattering, UV-visible spectroscopy, electron microscopy, and X-ray spectroscopies-with special attention paid to their strengths and weaknesses. Several specific examples of transition-metal nanocluster formation are then given, beginning with LaMer's classic sulfur sol system and including the Finke-Watzky mechanism Of Slow Continuous nucleation A -> B followed by fast autocatalytic surface growth A + B -> 2B. Finally, brief overviews of semiconductor nanoparticle preparations, solid-state nucleation studies-emanating from Avrami's work-and protein agglomeration mechanistic studies are also provided, as these processes are relevant, conceptually and in a general sense, to the field of transition-metal nanocluster nucleation and growth mechanisms. (C) 2007 Elsevier Inc. All rights reserved.