It is the purpose of this report to summarize the state of knowledge regarding the hydration of C//3A and C//4AF in the presence of calcium sulfate at ordinary temperatures. These interstitial phases in combination with calcium sulfate may comprise up to 20 percent of portland cement and their hydration reactions affect rheological properties, early strength gain characteristics, and durability. Within limitations, reported results are related to the hydration of the interstitial phases in portland cement. Although C//3A and C//4AF are used as models for the interstitial phases in portland cement, two factors may lead to differences in the hydration behavior of the pure phases and the interstitial phases in cement paste. First, in the manufacture of cement, extensive substitution may occur resulting in the formation of anhydrous aluminate and ferrite solid solutions of variable composition. Second, the interstitial phases in commercial cements hydrate in the presence of other phases. These factors may result in significant variations in the rates, and possibly, in the mechanisms of interstitial phase hydration in commercial cements.