Extreme operating and environmental conditions, such as high and varying load, high temperature, high or varying speed, and less than ideal lubricating fluids can cause excessive wear, damage, and even failure in conventional thrust bearings. This paper discusses the advantages of polycrystalline diamond (PCD) bearings operating in boundary, mixed-mode and hydrodynamic lubrication regimes. Experimental results demonstrate that PCD bearings survive and operate with low friction and little wear regardless of the lubrication regime. This is significant because bearings are commonly designed to function well in only one mode of lubrication. Conventional fluid film bearings, for example, are designed for operation in the hydrodynamic regime only, but PCD bearings can be designed to operate efficiently with hydrodynamic fluid film lubrication, transitory mixed-mode lubrication, or even boundary mode lubrication. The results discussed in this paper confirm the potential effectiveness of PCD bearings in applications with variable loads, speeds, frequent starts, stops and less than ideal lubricating fluids.