As structural design and analysis become more refined and as construction practices increase in efficiency there is an increased need for fundamental information on the behavior of concrete under loads other than static. Particularly there is a demand for more knowledge and understanding of concrete fatigue, which refers to the phenomenon of rupture under repeated loadings each of which is smaller than a single static load that exceeds the strength of the material. Fatigue is exhibited when a material fails under stress applied by direct tension or compression, torsion, bending or a combination of these actions. Metal fatigue has been under investigation for more than a century and a considerable amount of knowledge about it has been accumulated. But research on concrete fatigue is not only far less advanced, but also considerably less conclusive. Fatigue investigations of concrete have sought to determine the effect of repeated tensile, compressive and flexural loads on ultimate strength and strain or deflection. It appears that stress repetition and not the rate of strain is the critical factor in fatigue. Of particular interest to contractors, tests have shown that fatigue strength is vitally affected by age and curing. Concrete that is carefully cured and aged displays greater resistance to fatigue than concrete inadequately cured and aged. In addition, concrete of a rich mix and a low water/cement ratio has a slightly higher fatigue strength. Recent experiments do appear conclusive regarding the endurance limit of concrete. Many early tests assumed that the material possessed an endurance limit similar to most metals. However it now appears that plain concrete in flexure, given an acceptable level of stress, possesses no endurance limit at least with loads up to 10,000,000 cycles.