YBa2Cu3O7−x thin films deposited by laser evaporation were studied by transmission electron microscopy. The crystal structure and microstructure of the films depend sensitively on the oxygen partial pressure during deposition. Cooling conditions affect primarily the oxygen sublattice and thus determine electrical properties in the superconducting state. Deposition of technologically relevant thin films requires an oxygen partial pressure of 0.3 mbar during deposition and slow cooling in an oxygen atmosphere. Such films have a Tc of 90 K and a transition width of 0.6 K. Electron diffraction patterns yielded a relative lattice parameter difference (b − a)/b of 1.6%, from which the oxygen content can be determined with considerable accuracy. Extended crystal defects and second phases were analyzed by analytical and high-resolution electron microscopy. Films deposited at lower oxygen partial pressures (≍10−2 mbar) exhibit a strongly strained crystal structure with lattice planes heavily bent on an atomic scale. In these films evidence for a decomposition reaction of YBa2Cu3O7−x was found by the simultaneous presence of BaCu2O2 grains and Y2O3 precipitates. The films deposited at low oxygen partial pressure become superconducting when cooled slowly in an oxygen atmosphere. The onset of the broad transitions lies at 50 K.