NASA's Langley Research Center has published technical report TB 26-08, devoted to the analysis of complex oscillation modes in lightly damped structural systems. The authors note that such complex modes can arise in linear structures even at low overall damping levels. According to the description, this behavior manifests when two structural modes are closely spaced in frequency and coupled to each other through non-proportional damping. As this damping interaction intensifies, the character of the vibrational modes changes, which can be significant for analyzing the dynamic behavior of engineering structures, including aerospace systems. The work is presented from the perspective of mode interaction, allowing for a more detailed study of the mechanisms leading to the emergence of complex oscillation modes. Such research is important for more accurate modeling and prediction of structural behavior under real operating conditions, where damping is rarely perfectly proportional. The full version of the report is available in PDF format on the NASA website. For additional questions, readers can contact the study's author, Joel W. Sills, a researcher at Langley Research Center, at the provided email address. The publication is part of a series of NASA technical reports that disseminate the results of scientific and engineering research conducted by the agency to specialists and the scientific community.