Pasturelands in the United States span a broad range of climate, soils, physical sites, and management. Rather than treat each site as a unique entity, this diversity must be classified into basic units for research and management purposes. A similar system based on ecological principles is needed for temperate semiarid to subhumid to humid areas. Forage suitability groups (FSG; a system based on soil types) offer one approach for doing so, but are heavily based on expert opinion and management regime. A quantitative scientifically-based system like the ecological site description scheme (ESD) used in rangelands would provide a uniform approach to assessing all grazing lands, allow the identification of reference states for monitoring and assessment, and facilitate the creation of state and transition models suitable for pasture systems. We propose a new system based on a probabilistic hierarchical classification of ecologically-relevant climatic, topographic and edaphic variables, which will provide continuity in assessing ecological services across a broad spectrum of grazing lands.