Radar provides an all-weather, day-and-night imaging capability for tracking hazardous events, mapping Earth’s surface and studying subsurface processes—all with direct applications to studies of volcanic activity and hazards. Ground-based Doppler radars in common use for weather forecasting can track downwind dispersal of volcanic ash clouds, which threaten aircraft even thousands of kilometres from their source. Backscattering intensity images from synthetic aperture radars (SARs) in Earth’s orbit can effectively map fresh volcanic deposits (e.g. lava flows, mudflows, debris flows and tephra rafts) and other surface changes (e.g. ash deposition and fracturing) that might otherwise go undetected during a crisis. Time-series SAR images can be used to monitor and characterize volcanic processes, such as lava dome growth and landform changes resulting from explosive activity. The interferometric SAR technique combines the phase components of multiple SAR images to map subtle deformation of the ground surface and, through numerical modelling, to constrain the nature, location and shape of deformation sources (e.g. subsurface magma accumulation, hydrothermal system depressurization resulting from cooling or volatile escape and thermoelastic contraction of young flows). Combined with information from seismology, geodesy and geochemistry, radar in its various implementations enhances scientists’ ability to study volcanic processes, anticipate future eruptions and assess associated hazards.