Millipedes are a diverse and ancient group of poorly known terrestrial organisms. While recent advances in their taxonomy and distribution have occurred in some areas of the world, our knowledge about the distribution and ecology of many taxa in the Pacific Northwest is limited. We review the ecology of taxa we observed and present results from a field study relating millipede abundance and community composition to environmental conditions of geology, vegetation, and climate. Millipedes of southwest Washington State were surveyed in the spring and fall of 2005 and 2006 along twelve headwater streams in forested landscapes. Overall, we observed 10 families of millipedes, with confirmed identification of 15 species. Millipede community composition differed strongly between seasons and across sites. For each season, we report family-specific multiple regressions relating millipede abundance/presence to environmental conditions. Given the ecological importance of millipedes as detritivores, more information on taxonomy and environmental relationships is needed. This research provides insight into the patterns and distribution of riparian-associated millipedes in the Pacific Northwest.