Polygonal mirrors are useful devices for scanning light beams within certain boundaries of performance. This chapter aims to outline those boundaries as an aid to the system designer in selecting the most practical method of scanning in view of a particular need. Four common types of polygonal scan mirror have been developed: regular polygons; irregular polygons; inverted polygons; and pyramidal polygons. The major applications for regular polygons are: nonimpact computer output printers, laser inspection systems, infrared viewing systems and graphics facsimile systems. The symmetry of regular polygons enables the stacking of several polygons on an arbor for processing during manufacture. Entrance scanning is a term used to describe the use of a polygon to deflect a ray bundle, which after deflection is finally imaged by a lens or curved mirror. Passive correction schemes usually utilize additional optical elements to reduce polygon-induced tracking errors.