Recent tightening of particulate matter (PM) emission standards for heavy-duty engines has spurred the widespread adoption of diesel particulate filters (DPFs), which need to be regenerated periodically to remove trapped PM. The total impact of DPFs therefore depends not only on their filtering efficiency during normal operation, but also on the emissions during and the frequency of regeneration events. We performed active (parked and driving) and passive regenerations on two heavy-duty diesel vehicles (HDDVs), and report the chemical composition of emissions during these events, as well as the efficiency with which trapped PM is converted to gas-phase products. We also collected activity data from 85 HDDVs to determine how often regeneration occurs during real-world operation. PM emitted during regeneration ranged from 0.2 to 16.3 g, and the average time and distance between real-world active regenerations was 28.0 h and 599 miles. These results indicate that regeneration of real-world DPFs does not substantially offset the reduction of PM by DPFs during normal operation. The broad ranges of regeneration frequency per truck (3-100 h and 23-4078 miles) underscore the challenges in designing engines and associated aftertreatments that reduce emissions for all real-world duty cycles.