In this paper, the basic features of the novel field of the use of phase diagrams for the description of metastability will be outlined. Examples of experimental determination and calculation of metastable phase diagrams will be presented. In particular, metastable phase diagrams will be used to describe phase selection in cast iron (Fe-C) and stainless steels (Fe-Ni-Cr), showing the effect of quenching rate on solidification products. The extension of solid solubility by rapid solidification will be shown for the Au-Fe system, where giant magnetoresistance effects have been observed after suitable thermal treatments. Glass formation either by rapid solidification or by ball milling will be described in terms of metastable phase diagrams for Cu-Ti and Al-Ti. The formation of metastable compounds will be reported in Fe-B and Ni-B systems. Finally, the formation of nanocrystals from amorphous precursors will be described for Al-Ce and Al-Ni-Ce on the basis of calculated metastable phase diagrams.
