The processes of nucleation and growth of silver clusters under ion irradiation are investigated respectively in ion-exchanged glasses containing no clusters and in sputtered films, in which all the metal is precipitated before the irradiation. The studied samples were irradiated with He or Au ions of energy in the range of 1-100 MeV and the yields of precipitation or growth were assessed by combining measurements of luminescence and of optical absorption. The two processes result from electronic excitations and the nuclear collisions have insignificant effect. The nucleation rate is proportional to the product of the reduction cross section of Ag + ions by the ion fluence and the process involves the transient formation of Ag 32+ ions. Ag atoms must be released from clusters in order to permit the growth of some others in a matrix free of solute atoms (sputtered films) and this desorption is the stage limiting the growth rate. The desorbed atoms remain in solid solution when the density of electronic excitations exceeds a threshold value.