Existing work in the mechanical behavior of thin films focuses mainly on measurement of macroscopic properties without strong correlation to microstructural features. We used transmission electron microscopy (TEM) to characterize the microstructures of free-standing copper thin films both before and after monotonic tensile deformation in an ex-situ thin film tensile testing system, as well as during in-situ loading in the TEM. The defect structures contributing to plastic deformation were investigated with an emphasis on comparison to mechanisms known to operate in bulk copper. The thin film exhibited much lower ductility (approximately 1%) than that normally observed in bulk form (greater than 40%). The predominant plastic deformation mechanisms did not include the typical dislocation activity that occurs in bulk copper, but rather greatly inhibited dislocation interactions typical of stages I and II hardening only. The absence of those structures normally found in tensile-deformed bulk copper is attributed to the differences in characteristic sizes of features within the microstructure available for deformation in bulk versus evaporated thin film material, that is, grain size and film thickness. The thin film ductility is an order of magnitude lower than what has been observed in bulk, ultrafine-grained copper, implying that a pure thin film effect on ductility exists and is significant. Microstructural features both near to and far from the fracture surface regions will be presented.