Mirrors come in all shapes and sizes, but from an engineering point of view, just what is a mirror? There are large mirrors such as the HST’s highly structured lightweight primary mirror, the deployable segmented 8 m James Webb Space Telescope mirror, the segmented 10 m mirror of the Keck telescope, and the active thin-shell primary mirror of the 8 m Gemini and Subaru telescopes. There are small scanner mirrors, flip mirrors in cameras, and cooled mirrors in high-energy systems. Is the mirror a reflecting surface or the structure that contains the reflecting surface? A good definition is that the mirror consists of a reflecting surface and whatever it takes to support that surface. A mirror is then an optomechanical system in itself.