The use of silica fume as a mineral admixture for the production of high strength high performance concretes is gaining importance in recent years. The present paper is an effort towards a better understanding of the efficiency of silica fume in concrete. It was observed from an evaluation of the data available in literature that the efficiency of SF in concrete was not a constant at all percentages of replacement. It was proposed that the "overall efficiency factor" of SF can be assessed in two separate parts, the "general efficiency factor" - a constant at all the percentages of I replacement and the "percentage efficiency factor" - varying with the replacement percentage. A comparison of the efficiencies obtained from the earlier data with studies on a "Lower Grade Silica Fume" in the laboratory show that the proposed values of efficiency are of lower bound and it is possible to achieve even higher efficiencies with proper mix proportioning. © 1995.