The average person would probably feel mildly alarmed on hearing that dental fillings contain locked-in stresses that can lead to fracture and to breakage. Eliminating such stresses from materials like dental fillings whenever possible seems highly sensible. However, it is difficult to work a rigid material without introducing such permanent, residual stresses. Most common manufacturing operations such as turning, grinding, and welding can create them. But sometimes residual stresses can be beneficial, and manufacturers may purposely introduce them. The valve springs of automobile engines, for example, are routinely treated by an operation called peening: the bombardment of a part with sand or small beads of metal or glass. Peening imparts a compressive stress field to the surface layer of the material, and this field slows the growth of cracks due to corrosion or local deformation-the field literally forces surface cracks closed. Springs treated in this manner last ten times as long as untreated springs. Many such stress-related effects and the recipes for obtaining them have long been known to machinists and other artisans. What is new today is scientists' ability to measure stress fields directly, rather than inferring their nature from observed warpage and breakage in a process of trial and error. Several trends in engineering design make this new analytical competence extremely important. One of the trends is the proliferation of nontraditional materials. The best way to work a sword blade is known, but no comparable body of lore exists for ceramic transducers or semiconductor wafers or composite airfoils. A second trend is the trimming of safety margins. Because residual stresses can add to the loads a structure must bear, bridges and other load-bearing structures have traditionally been built with several times the strength estimated as necessary to carry the maximum expected load; the lavish use of materials required by this practice is increasingly uneconomic. A third trend is the reduction in scale of many engineered systems. Residual stresses play a much more important role in the microscopic metal parts on the surface of a chip than they do in a thick electrical cable.