Xerographic toners are typically blended with additives for adhesion control in development and transfer processes. Surface additives, 10–100 nm in size, are used to space toners away from the electrode surfaces, thereby lowering adhesion forces. However, in a developer housing, additives get buried into the toner over time due to the repeated mechanical stresses encountered. This is referred to as toner aging. Aged toners can have significantly higher adhesion forces and often perform poorly in development and transfer. In this article, we will discuss models for estimating the surface additive coverage distribution on toners in the developer housing and its impact on development and transfer performance. The use of models to evaluate options for the control of the toner material state will be discussed.