Tunable Metamaterials The electromagnetic response of metamaterials gives rise to exciting phenomena such as cloaking, negative refraction, and perfect lensing. Their response, however, tends to depend strongly on resonant effects, thereby limiting the application bandwidth. Driscoll et al. (p. 1518 , published online 20 August) combine a split ring resonator array with the phase change material, VO 2 , to form a metamaterial in which the response can be tuned. The heat-induced phase change of VO 2 from an insulator to a metal alters the response of split-ring resonator, and, because it displays a hysteresis, the device can retain a “memory” of the induced change. The results may lead to a flexible method for achieving metamaterials operating over a wide bandwidth and to novel switching applications.