In my experience, washers are used in under the turning element. For example, if you have a bolt and nut, typically the washer is under the nut, which is turned. This is to prevent, as mentioned arto, gauling. Also, the nut and washer surface should be lubricated (if they are not coated bolts being used for the first time, coated being a low friction coating) for flange bolts to reduce the friction between them, and thereby increase the bolt preload achieved by a given torque. Also, as mentioned by uncleysd, the washer should be hardened, note though it should be through hardened, not simply surface hardened. I disagree with rjcj9. If the bolt is suffiently preloaded, it is not subject to fatigue. The cyclic load reduces the contact stress between what is being bolted together. Generally (and there are exceptions), the bolt stress does not change until the length of the bolt changes, which means the contact surfaces separate. So you have to overcome the bolt preload before bolts experience cylic loads. A more detailed assessment would reveal, though, that bolt load can be increased or decreased by external loads in some situations, such as raised face flanges. With a raised face flange, an external tension load can, for example, rotate the flanges and cause a decrease in bolt load because the length of the bolt is decreased by flange rotation. As mentioned by haggis, another reason for washers is for oversize holes.