This chapter deals with special chemicals that can be used as cement additives. There are two basic kinds of activities in cementing, namely, primary and secondary cementing. Primary cementing fixes the steel casing to the surrounding formation. Secondary cementing is used for filling formations, sealing, or water shutoff. The performance can be varied by the addition of additives with regard to various aspects. Depending on the application of the cement, a wide variety of additives can be incorporated. These include accelerators, retarders, dispersants, extenders, weighting agents, gels, foamers, and fluid loss additives.