THIS CHAPTER IS THE FOURTH OF A SERIES DEALING with chemical reactions of aggregates in concrete. The first version, entitled “Chemical Reactions,” was written by W. C. Hansen and was included in ASTM STP 169A, published in 1966. It dealt with several types of chemical reactions known at the time to occur in concrete, including alkali-silica reactivity. The second version was written in 1978 by Sidney Diamond for ASTM STP 169B and was entitled “Chemical Reactions Other Than Carbonate Reactions.” It specifically excluded alkali-carbonate reactivity and emphasized alkali-silica reactivity. The third version, written by the present author, covers only “Alkali-Silica Reactions in Concrete.” This fourth edition reviews and updates the topics as addressed in cement and concrete articles and introduces new technology that has been developed.