Calcium chloride offers many advantages that make it popular as a concrete accelerator. It causes a substantial increase in early strengths and it speeds rate of set. It is readily available. It dissolves easily in water. Its behavior with most ingredients of concrete and under a wide variety of conditions has been studied and reported over a long span of years, and its effects have been generally understood though in recent years increasing evidence has accumulated that calcium chloride or other source of chloride ions can cause serious problems in concrete. This article attempts to put findings in this area in perspective even though, as in many areas of concrete technology, there are side divergences of opinion. Many properties of concrete are affected to various degrees by calcium chloride: the heat of hydration at 1 day increases 30 percent; tensile strength at 28 days is slightly decreased; flexural strength at 7 days is increased by 10 percent; flexural strength at 28 days is a 0 to 15 decrease; and volume change shows a 0 to 15 increase. We now know that chloride greatly increases the likelihood of corrosion. We also know that, under conditions where moisture penetrates to the steel, only the chemical basicity of portland cement pasts prevents corrosion. Unless large amount of chloride are present, the basicity factor usually wins out, at least during the relatively short haul of 20 to 30 years. However, concrete slowly carbonates upon exposure to air and this carbonation destroy the basicity of concrete. The prognosis is bleak when that basicity is lowered and water and oxygen penetrate to the level of the steel. The results can be disastrous if chlorides are present. One factor that has not been widely publicized but merits major emphasis is that the chloride concentration often varies in different portions of a reinforced concrete structure. An example of such an affect is the cast-in-place reinforced columns of a multistory building. After three years of service cracking over and in line with reinforcing steel was permanent in only the chloride-containing portions of the columns. Cover over the steel in several instances was 1 inch and in other instances 3 and one-half inches. The reason was differential chloride content.