During cementing operations, the hydrostatic pressure of the cement slurry column is utilized to control the reservoir pressure. However, when the well extends through weak formations, it is often necessary to lighten the hydrostatic pressure of the slurry by lowering the density to avoid hydraulically fracturing the reservoir rock. Densities of cement slurries can be lowered by adding pozzolanic lightweight additives such as fly ash. Fly ashes have found utility in numerous oil and gas well cementing applications, namely: 1) fly ash can be dry-blended with regular Portland cements or hydrated lime for use in primary cementing; 2) lightweight cement compositions can be formulated by adding lime and finely ground quartz to fly ash/cement mixtures; and 3) an ultralight-weight pozzolanic microsphere cement, with densities as low as 0.96–1.64 g/cm3, can be formulated by blending Portland cements with hollow microspheres isolated from fly ash. This paper discusses the physical and chemical properties of each of these cement slurries, along with their relative advantages and disadvantages. Areas of future research are also discussed.