Rotary mirror scanners are often purchased as "standard" motorized spindles to whose shafts a range of mirror-faceted polygons can be fixed. This approach usually offers low unit costs and has proved satisfactory for many optical scanning systems, but for more exacting requirements these units can be wasteful of energy or space or can have inadequate performance. This chapter looks at the windage of rotating polygons, an aspect of performance normally difficult to estimate, and describes a proven method of predicting the windage for any rotating scanner. The gas atmosphere in which the rotor turns has an important effect on windage. Reductions in windage can be obtained by the use of modifications to the rotor, typically rounding of the rotor corners or the fitting of side cheeks. The windage of rotary scanners can be affected by many interdependent variables of the mirror polygon and its surroundings.