Large-amplitude rain-wind induced cable vibrations is a source of concern for cable-stayed bridges in the United States and worldwide. Enhanced cable damping is a crucial factor in the control of stay cable vibrations including the rain-wind vibration phenomenon. In a research project sponsored by the National Cooperative Highway Research Program (NCHRP), the effectiveness of a number of internal and external methods for damping improvements in stay cables was investigated. Experimental work was based on comparative assessments performed on two one-seventh-scale models of a `representative' stay cable. Each cable model was approximately 45 ft long. The methods investigated included conventional and damping grout fillers, conventional neoprene rings (washers), polyurethane filled guide pipes, damping tapes, and external tuned mass dampers. In this paper, a discussion of different damping treatments and their effectiveness is presented. The degrees of effectiveness varied considerably for the various treatments. Of the methods tested, the tuned mass damper for stay cables was found to be the most effective method.