Book excerpt: This work was motivated and focused on the growth mechanism, microstructures and interfaces of barium titanate, BaTiO3 (BTO), based epitaxial thin films. Transimission electron microscopy (TEM), high resolution TEM (HRTEM), electron diffraction (ED) have been mainly employed in this study for microstructure investigations of BTO-based epitaxial thin films with the assistance of other materials characterization methods, such as X-ray diffraction (XRD), atomic force microscopy (AFM), Nanoindentation testing, etc. As a basic studying model, epitaxial BTO films were first deposited on the (001) MgO substrate using RF magnetron sputtering to provide fundamental knowledge of the film growth for BTO based thin films. The epitaxial BTO films show a tetragonal crystal structure (a = 4.02 Å and c = 4.11 Å) with epitaxial nanodomains induced from the rough film/substrate interface due to the modification of the substrate surface characteristics (steps, terraces, and kinks) during the deposition process. Two new phases of - calculus [gamma]y (Ba10TiO26) and Hß (BaTi8O24) were formed on the MgO substrate using RF magnetron sputtering. The atomic structures of the two new phases were determined and examined. The [gamma]y phase has a monoclinic structure, Cm, a=16.49 Å̊̊, b=8.94 Å, c=3.94 Å, y=77°. The Hß phase has a monoclinic structure, Cm, a = 17.88̊Å̊, b = 7.21Å, c = 3.94 Å̊̊̊, y =82°. Both [gamma]y and Hß phases have four different oriented domains. The orientation relationship between domains and MgO substrate is discussed. Initail study shows that these new phases possess novel and interesting properties. The doped BTO films with different dopant (Sr2 and Zr4) fabricated using pulsed laser deposition (PLD) have been studied for doping effects, interface effects and film growth mechanism. The (Ba, Sr)TiO3 (BST) epitaxial films grown on 1.2° and 5.3° miscut substrates consist of lager commensurate domains, and correspondly show higher dielectric constant and dielectric tunability, and lower average modulus and hardness than those of the films grown on the 3.5° miscut substrate. The results suggest that the differences on the interfacial structure have a tremendous effect on the properties of the films. The investigation on epitaxial Ba(Zr, Ti)O3 (BZT) and 2% Mn doped Ba(Zr, Ti)O3 (Mn:BZT) thin films grown on MgO substrate revealed the formation of twin-coupled domains on the epilayer by sharing their {111}/{110} planes as common planes. The structure evolution from epilayer to nanopillars is accomplished by alternatively introducing {111} and {110} twin boundaries, resulting in gradual shrinking or enlarging of the lateral size of the epitaxial grains/twin coupled nanopillars. The formations of twin domains, sharing planes and structure transition mechasim have been discussed in detail. Eventually, the microstructures and layer interfaces of [Mn:BZT//Mn:BST]N multilayer films consisting of 2% Mn doped (Ba, Sr)TiO3 (Mn:BST) and Mn:BZT layers on MgO (001) substrates with various deposition rates, deposition period numbers N were studied to provide an integral view of the formation of multilayer films in BTO system. All the [Mn:BZT//Mn:BST]N multilayer films present epitaxial quality and induced {111}/{110} twin-coupled domains initially formed at different film thickness/layers. The film/substrate interface has an important role for the formation of twin domains and the proportion of twin domain structures in the entire films. The layer interfaces between Mn:BST and Mn:BZT with period compression and extension strains were observed and discussed. The microstructure evolution and structure-related nano-mechanical properties of the multilayer films have been systematically investigated.