External additives for toners such as hydrophobic fumed silica and hydrophobic titania have been widely used for improvement of toner performance. Toners formulated for use in laser printers and PPC (Plain Paper Copiers) are being developed, aimed at high speed copying, high image quality and downsizing. To meet the challenges of more demanding requirements, toners are designed utilizing both smaller and larger particle size external additives to achieve better flowability, higher durability, higher image density and stable electrostatic charge under ambient conditions. Usage levels are also increasing. In addition to improved overall toner flow and charge properties, this trend is associated with improving cleaning properties such as reduced fogging and adherence of toner to paper.As such properties can be influenced by external additives, it is important for the toner formulator to know which characteristics need improvement in the already design stage.This paper describes the design and the selection of external additives. It will address the influence of mixing time of external additives on the tribo-electrostatic charge (T-ESC), charge distribution, and flowability of toners. In addition, the influence of surface chemistry of external additive on the admix performance is discussed. Finally, experimental results of the performance of color toner mixed with several kinds of external additives are reported.