Abstract The well cementation design process includes slurry design, spacer design, hole conditioning program design, casing attachments selection, and placement procedure design. This paper is concerned with slurry design and more particularly, the effects of deviations in mixed slurry density from the design point. Concern over these effects has been increasing in recent years. In 1987 an API Task Group in the Eastern Hemisphere Cementing formed a work group to study "the effect of poor density control on cement slurry properties " This group also investigated measurement and control of density. There were wide differences of opinion as to how well density was being controlled in the field and also what was necessary. Additional data and analysis will be presented which will help clarify some of the density slurry property relationships and therefore provide additional means to make rational decisions relative to density control specifications. Density as a slurry property is significant to wellbore hydrostatic pressure. However, deviations from design density may cause changes in all other slurry properties which may or may not cause future well problems. The relationships defining slurry property dependence on slurry density are important when establishing meaningful density control limits. However, closer limits generally mean higher mixing system and operation costs. Thus, there is an economic motivation for understanding how deviations effect slurry properties. The knowledge of the cost to obtain a specific density tolerance can be weighed against its benefit. More appropriate specifications can be developed to match job requirements. The end result could be a change in the way slurry properties are specified. Knowledge of the sensitivity of the designed slurry to density variation will make selection of surface mixing equipment more meaningful. Batch mixing gives the ultimate in control but cost more. Offshore, batch mixers take up valuable rig space and add weight.