This paper summarises new results which illustrate some important concepts in phase transformation and compound formation under ion irradiation. Firstly, the amorphization behaviour in typical semiconductors and metals is contrasted by examining different modes of nucleation of the amorphous phase. Secondly, silicon dioxide formation within amorphous silicon during low temperature, high dose oxygen bombardment is examined. Finally, we illustrate the formation of epitaxial Ni 3Si on Ni(100) by high dose Si implantation and subsequent annealing.