Molecular clusters are sometimes seen to undergo the same phase changes observed in bulk systems, but at much deeper supercoolings and at enormously higher rates. Under other conditions they can be seen to transform spontaneously to crystalline structures not found in the bulk, for reasons that are beginning to be understood. Nucleation rates in clusters have been measured experimentally in supersonic beams by electron diffraction and computationally by molecular dynamics techniques. The two approaches are proving to be complementary and effective research tools. Despite the fact that a large proportion of the molecules in clusters lie in surface layers, compensating characteristics make clusters attractive objects of inquiry. Although surface layers are poorly representative of ordinary phases, cluster cores have been shown by experiment and computation to have properties close to those of bulk matter. Advantages and limitations of clusters in research on the dynamics of phase changes are reviewed, and illustrative examples are presented.