Phase transitions are critical in many of the new technologies of interest to chemical engineers. Applications to materials processing and pharmaceuticals manufacture are among the uses for phase transition dynamics investigated with the methods of population balance modeling for clustering processes. The underlying phenomenon of clustering occurs during condensation processes such as crystallization from solution. Self-assembly of molecules or supramolecules is likewise spontaneous clustering, often through non-covalent interactions. Vapor-liquid and solid-liquid condensations usually involve nucleation from a metastable state, accompanied by particulate growth and Ostwald ripening (with denucleation). The nucleation process is bypassed under certain conditions, such as in spinodal decomposition, glass transition, and gelation. It is proposed that the unifying concept for all these transformations is cluster kinetics and dynamics. Using population balance modeling, we outline how phase transition processes can be quantitatively modeled by cluster size distributions evolving in time. We also discuss how generic cluster processes apply to granular systems, synchronizing oscillators, and polymorphic transformations.