The Radar Imager for Mars' Subsurface Experiment (RIMFAX) is a Ground Penetrating Radar on the Mars 2020 mission's Perseverance rover, which is planned to land near a deltaic landform in Jezero crater. RIMFAX will add a new dimension to rover investigations of Mars by providing the capability to image the shallow subsurface beneath the rover. The principal goals of the RIMFAX investigation are to image subsurface structure, and to provide information regarding subsurface composition. Data provided by RIMFAX will aid Perseverance's mission to explore the ancient habitability of its field area and to select a set of promising geologic samples for analysis, caching, and eventual return to Earth. RIMFAX is a Frequency Modulated Continuous Wave (FMCW) radar, which transmits a signal swept through a range of frequencies, rather than a single wide-band pulse. The operating frequency range of 150-1200 MHz covers the typical frequencies of GPR used in geology. In general, the full bandwidth (with effective center frequency of 675 MHz) will be used for shallow imaging down to several meters, and a reduced bandwidth of the lower frequencies (center frequency 375 MHz) will be used for imaging deeper structures. The majority of data will be collected at regular distance intervals whenever the rover is driving, in each of the deep, shallow, and surface modes. Stationary measurements with extended integration times will improve depth range and SNR at select locations. The RIMFAX instrument consists of an electronic unit housed inside the rover body and an antenna mounted externally at the rear of the rover. Several instrument prototypes have been field tested in different geological settings, including glaciers, permafrost sediments, bioherme mound structures in limestone, and sedimentary features in sand dunes. Numerical modelling has provided a first assessment of RIMFAX's imaging potential using parameters simulated for the Jezero crater landing site.