Currently, there is no comprehensive terrestrial ecosystem classification for the central Rocky Mountains of the United States. A comprehensive classification of terrestrial ecosystems in a mountainous study area in northern Utah was developed incorporating direct gradient analysis, spatial hierarchy theory, the zonal concept, and concepts of diagnostic species and fidelity, together with the biogeoclimatic ecosystem classification approach used in British Columbia, Canada. This classification was derived from vegetation and environmental sampling of both forest and non-forest ecosystems. The SNOwpack TELemetry (SNOTEL) and The National Weather Service (NWS) Cooperative Observer Program (COOP) weather station network were used to approximate climate of 163 sample plots. Within the large environmental diversity of the study area, three levels of ecosystem organization were distinguished: (1) macroclimatic - regional climate; (2) mesoclimatic, accounting for local climate and moisture distribution; and (3) edaphic - soil fertility. These three levels represent, in order, the L+1, L, and L-1 levels in a spatial hierarchy. Based on vegetation physiognomy, climatic data, and taxonomic classification of zonal soils, two vegetation geo-climatic zones were identified at the macroclimatic (L+1) level: (1) montane zone with Rocky Mountain juniper and Douglas-fir; and (2) subalpine zone with Engelmann spruce and subalpine fir as climatic climax species. A vegetation classification was developed by combining vegetation samples (relevés) into meaningful vegetation units. A site classification was developed, based on dominant environmental gradients within the subalpine vegetation geo-climatic zone. Site classes were specified and a site grid was constructed. This site classification was coupled with the vegetation classification. Each plant community was associated with its environmental space within the site grid. This vegetation-site overlay allowed ecosystems to be differentiated environmentally and a structure, combining zonal, vegetation, and site classifications, forms a comprehensive ecosystem classification. Based on assessment of plant communities' environmental demands and site vegetation potential, the comprehensive classification system enables inferences about site history and successional status of ecosystems. This classification is consistent with the recent USDA, Forest Service ECOMAP and Terrestrial Ecological Unit Inventory structure and may serve as a valuable tool not only in vegetation, climatic, or soil studies but also in practical ecosystem management.