The often-reported premature failures of expansion joints of long-span steel bridges call for a better understanding of their performance in service conditions. This paper presents the long-term monitoring and analysis of displacements of expansion joints, with emphasis on the influence of viscous dampers that were installed to mitigate seismic responses of the bridges. Data from the three long-span steel bridges in Chinathe Sutong Bridge, the Runyang Suspension Bridge, and the Jiangyin Bridgeare compared. Decomposition of displacement signals in various frequency bands reveals that temperature-induced movements contribute to only a small portion of the total cumulative displacements. Because of the small but rapid movements caused by vehicle/wind loads, the cumulative displacements of long-span steel bridges may far exceed those caused by temperature changes. According to the monitoring data, the existence of viscous dampers may reduce the vehicle/wind-induced displacements but have no significant influence on the temperature-induced movements. Finite-element analyses are conducted to further validate the influence of viscous dampers on displacements of expansion joints. (C) 2014 American Society of Civil Engineers.