Thin circular saws can suffer from a vibration phenomena called critical speed instability. At the critical speed, a resonant condition occurs where a saw can snake slowly from side-to-side producing unacceptably large sawing variation. To avoid this problem, most circular saws operate at 10 to 15 percent below the first critical speed. Unfortunately, this practice limits reductions in saw plate thickness and corresponding improvements in lumber recovery. Recently, several mills have been able to operate guided splined-arbor saws above critical speed. These supercritical speed saws offer significant sawing performance improvements by allowing both high recovery and high production rates. The use of these saws remains rare, however, and very little knowledge is available to guide mills that want to investigate this promising area. This report describes laboratory tests involving two supercritical speed saw configurations which are operating successfully in industry. Detailed descriptions of these configurations are given and guidelines for use are described. Idling and cutting tests were completed to characterize vibrational behaviour and demonstrate sawing performance levels. Tests were done to investigate the effect of changes in saw tensioning, saw tooth design parameters, saw and feed speeds and saw plate thicknesses. Test results confirm that for the sawing configurations tested, operating speeds can be found in the supercritical speed region where stable idling behaviour allows successful sawing. As with conventional saws, sawing accuracy is best at lower feed speeds. However, by operating at supercritical speeds, acceptable sawing accuracy can be achieved at higher feed speeds than are possible using conventional saws. Saw tensioning allows further increases in saw and feed speeds, but is not essential for supercritical speed operation. Changes in saw thickness strongly affect sawing performance levels and lumber recovery. Thinner saws have higher sawing variation and must operate at lower feed speeds than thicker saws. Curves showing the relationship between saw plate thickness, feed speeds and sawing accuracy are presented which can be used to assess the economic benefits of the supercritical speed saws that were tested. Supercritical speed circular saws offer considerable potential to improve sawing performance and increase lumber recovery. These saws allow reductions in saw kerf widths while maintaining high feed speeds and acceptable sawing accuracy.