This chapter reviews the transformation of solution-derived ferroelectric thin films from the as-deposited to the crystalline state that occurs during heat treatment. Reaction chemistry and pathways associated with the elimination of organic constituents from the film are discussed, as are the thermodynamic and kinetic aspects of the nucleation and growth processes that lead to crystallization. Related topics discussed in this chapter include structural evolution during pyrolysis, thin film densification processes, the role of interfacial layers on film orientation, and control of thin film crystallization behavior and microstructure. A focus of the chapter is the consideration of solution chemistry effects on pyrolysis, densification and crystallization behavior and illustration of the magnitude of such effects on thin film microstructure.