The undercooling of liquids is observed often, but is restricted by the catalysis of heterogeneous nucleation sites in contact with the liquid. Containerless liquid processing and liquid dispersal into fine droplets yields an effective nucleant isolation which permits a deep undercooling approaching 0.3–0.4Tmbefore the onset of solidification. At high undercooling the nucleation of an equilibrium phase may be superseded by metastable product structures. Often processing variable such as melt superheat, liquid surface films, powder size and cooling rate can be used to control the undercooling and produce a transition in phase selection kinetics. In this case the use of metastable phase diagrams is important for the interpretation and prediction of solidification products such as supersaturated solutions, metastable intermediate phases and glasses. At the same time a consideration of the competitive nucleation and growth kinetics and solidification thermal history is essential in the analysis of solidification microstructure. These features highlight a number of solidifcation reaction paths and new opportunities for the development of microstructure control.