Grazed pastures in the northeastern USA contain far more than planted forage species. These species may contribute to forage production, but they may also detract from forage production or palatability. As the first step toward identifying the role of plant diversity in forage systems, we collected data on vegetation, topography and soils for 130 pastures on 44 farms between 1998-2005. Farms were located from Maryland to Maine, and all were grazed. We found 310 identifiable plant species. Bluegrass, white clover, orchardgrass, tall fescue and dandelion were the most common. Most species were rare, and nearly 25% were found only once. The average pasture contained 32 species (9-73 species per quarter-acre plot). Most species found in pastures are not considered to be forage species. Forage species richness was much less variable, with an average of 8 species (3-12 species per plot). Forage species made up more than 75% of the plant cover in over half of the pastures sampled. The average pasture had 6% bare ground, 67% grass, 21% legume and 23% forb cover. We used stepwise regression to identify the variables related to important characteristics of the pasture plant community: number of species and total cover, and number and cover of grasses, legumes, forbs and forage species. Latitude and longitude were important. Slope and elevation were sometimes important, but aspect was not. Soil texture was important, but soil organic matter and nutrient availability were not. Pasture plant communities are far more diverse than might be expected, but diversity and composition appear to be influenced more by management than by site factors.