This article will explore why fumed silica is so widely used and valued by the formulator. The origin and various types of fumed silica will be described. The property modifications that are possible will be identified with the focus being on thixotropy and rheological properties. Starting formulations will also be provided for several products. Fumed silica is a popular filler that is used in many adhesive and sealant formulations. Even though fumed silica is not a very good extender for minimizing cost, modifying thermal expansion coefficients, or significantly improving cured strength, it is one of the most widely used fillers in a variety of applications from structural adhesives to pressure sensitive adhesives. Fumed silica, an amorphous silicon dioxide, is a versatile, efficient additive used in adhesive and sealant formulations primarily for flow control and thixotropy. Fumed silica has long been the dominant thixotrope employed in the adhesive and sealant industry.