Imaging radars are all-weather instruments that can image planetary surfaces regardless of local atmospheric or solar illumination conditions. Radar images provide information about surfaces that are complementary to the chemistry usually inferred from visible and infrared images. Instead, radar images are strongly influenced by surface roughness and geomorphology, and to a lesser extent by the bulk electrical properties of the surface. This chapter describes the basic principles of high-resolution synthetic aperture radars (SARs), as well as advanced SAR implementations. Radar polarimetry provides information about surface roughness and electrical properties, while radar interferometry allows the measurement of surface topography and surface deformation following events such as earthquakes or volcanic inflation. Radar imagers have returned spectacular information about the surfaces of both Venus and Titan, bodies with dense, opaque atmospheres that are difficult to image using traditional camera systems. Examples of both planetary and Earth observations with SAR are discussed to illustrate the utility of these images.