The microstructure of Pb(Zr, Ti)O3 (PZT) and PbTiO3 (PT) thin films deposited by the sol-gel method and chemical vapor deposition, respectively, were examined by transmission electron microscopy (TEM). Domains with ∼7 and ∼20 nm widths were found for the PZT and PT thin films, respectively. The traditional parallel twin or wedge-type structures found in bulk ceramics have been observed in thin films. Differences between observed grain sizes and previous studies of similar compounds (in bulk form) are accounted for by geometrical considerations related to crystallographic factors. Finally, a classification scheme for domains in PZT and PT thin films based on these and other published results of several researchers is presented. Domain sizes varied according to three categories: mono-domains (2–50 nm in diameter), domains in spherulite lamellae (28–130 nm wide), and twins in conventional large grains (5–150 nm wide). The mono-domains are related to small grain sizes, while the lamellae are a function of the nucleation and growth associated with sol-gel processing.