Filter regeneration systems require an accurate and reliable measurement of diesel particulate (soot) accumulation. This paper describes recent advances in the development of a device that uses radio-frequency (RF) technology to measure filter soot loads. A one-point calibration method is presented that provides a means to significantly reduce filter system development costs. Results from both on-vehicle and laboratory testing programs are discussed. The on-line monitoring of catalytic and energy-addition regeneration processes is also described.