THIS WRITER IS INDEBTED TO PRIOR AUTHORS OF this chapter [1–4] because their work provides the background for this version. I have borrowed from their ideas and specific words. However, following the format provided by Cain [4], this chapter will present less discussion of the use of these materials in concrete and more on the characteristics of the various materials and the significance and use of their acceptance tests. For discussion of the use of these materials in concrete, the reader is referred to three reports from the American Concrete Institute (ACI): ACI 232.1R “Use of Natural Pozzolans in Concrete” [5]; ACI 232.2R-96 “Use of Fly Ash in Concrete” [6]; and ACI 234R-96 “Use of Silica Fume in Concrete” [7]. Updated reports on these materials are currently being prepared by the appropriate ACI Committees.