It is demonstrated that polycrystalline thin films of highTcsuperconducting YBa2Cu3O7−xcan be grown with clean grain boundaries, i.e., without a boundary layer of segregation or different phase. In clean stoichiometric samples, angular misorientations of grains may be the origin of weak link behavior. High‐resolution scanning transmission electron microscope images of films grown on ZrO2and MgO by reactive evaporation, reactive sputtering, and laser ablation show atomic lattice images of clean grain boundaries. X‐ray microanalysis with a 10 A˚ spatial resolution also indicates no composition deviation at the grain boundaries. Grain sizes and epitaxial relations of samples prepared by different methods are characterized.