The effects of donor (Nb, Ta) and acceptor (Na, Mg, Fe) dopants on the crystallization mechanism of PZT thin films were investigated. The parameters which control microstructure development were found to be different for donors and accepters. Lead stoichiometry was found to be the critical parameter for donor doped films. Donor substitutions in the perovskite lattice ABO(3), require a compensation by creation of A-site vacancies, Pb1-x/2(Zr,Ti)(1-x) NbxO3, in order to maintain electroneutrality. In the pyrochlore lattice A(2)B(2)O(7), however, compensation might occur either by creation of A-site vacancies, as in the perovskite Pb1-x/2(Zr, Ti)(1-x)Nb-xO(6)square' or by an increase in oxygen stoichiometry, Pb2+ (Zr, Ti)(1-x)Nb-4+(x)5+O6O'(x/2)square'(1-x/2), or by formation of a monoclinic structure with A to B ratio (1:3) Pb1-x/2(Zr, Ti)(3-x)NbxO7. Second phase retention is observed in the last two cases. Lead content controls which of the three stoichiometries will form. For acceptor dopants, it was found that the affinity of the dopant element to form pyrochlore phases with the host ions of PZT and the site of doping are critical parameters for microstructure development. The solubilities of accepters were found to be much higher in PZT films than in ceramics. This result leads to the hypothesis that the formation of holes versus oxygen vacancies is favored in acceptor doped films. The role of metallization on crystallization of doped films is discussed.