For more than three decades, extensive research work has been devoted to the unique properties of clusters. They are made of a small number (or nuclearity) of atoms or molecules only, and therefore constitute a new state of matter, with specific properties. New methods have been developed in physics and chemistry for their synthesis, their direct observation, the study of their properties, and of their crucial role in number of processes, such as phase transition, catalysis, surface phenomena, imaging. Owing to its specific approach, radiation chemistry offered first the opportunity to reveal the existence of nuclearity-dependent properties of clusters. Pulse radiolysis has then proven to be a powerful method to study the mechanisms of cluster formation, induced by irradiation as well as by chemical reduction, and the reactivity in solution or at the interfaces. (c) 2005 Elsevier B.V. All rights reserved.