Rotating Machinery, Optical Methods & Scanning LDV Methods, Volume 6: Proceedings of the 39th IMAC, A Conference and Exposition on Structural Dynamics, 2021, the sixth volume of nine from the Conference brings together contributions to this important area of research and engineering.  The collection presents early findings and case studies on fundamental and applied aspects of Structural Health Monitoring, including papers on:Novel TechniquesOptical Methods,Scanning LDV MethodsPhotogrammetry & DICRotating Machinery