We describe here the use of imaging radar interferometry for remote sensing studies of volcanoes. Interferometric radars measure topography precisely, map surface cover changes such as active lava flows, and resolve extremely small deformations of the surface over large areas. First we present a tutorial on the radar interferometry technique. We then describe how these detailed measurements of surface processes may be further analyzed to infer volume or other changes at depth and thus provide a window through which to view subsurface activity. Because the subsurface transport of magma within a volcano is an important indicator of potential (or ongoing) activity, the ability to monitor deformations when they are slow or small, and often aseismic, could provide advance warning of future activity. Because radar measurements are not affected by clouds, radar is a valuable means to gather information in challenging, and even dangerous, environments spread over the entire Earth. In this chapter we present the technique of radar interferometry in a tutorial manner, give several example analyses of active volcanoes, and discuss its usefulness for the study of volcanoes in a variety of environments.