Technical Abstract: Pasture lands are an important facet of land use in the northeast (NE) United States (U.S.), yet little is known about their diversity. Pasture lands may become even more important in the NE as farmers increasingly convert crop land to perennial grazing pasture as an alternative way to supply forage to livestock. Because little information exists about the ecology of NE pastures, we designed a vegetation survey to answer some fundamental questions about their species diversity. Our objectives were to document plant richness across a broad spectrum of NE pastures using an intensive, multi-scale sampling method and compare the richness of NE pasture lands with: 1) grazed grasslands in the western U.S., and 2) abandoned agricultural lands in the eastern U.S. that had not been grazed. We also wanted to learn whether environmental (soils or climate) or land management variables could help explain patterns of richness. A total of 17 farms, encompassing 37 pastures, were sampled in New York, Pennsylvania Vermont, Maryland, Massachusetts, and Connecticut during July and August, 1998. We positively identified a total of 161 different plant species across the study region. Species richness averaged 31.7 +/- 1.1 on pastures and was similar to grazed grasslands in the western U.S. at both 1 m2 and 1000 m2 scales. Infrequent, transient species that were mostly perennial and annual forbs accounted for ~90% of the species richness. Environmental variables (soils and climate) and land management histories were generally poor predictors of plant richness at large spatial scales. At the 1000 m2 scale patterns of species richness are best explained by the species diversity of soil seed banks or seed rain, and stochastic recruitment of these species into existing vegetation.