Many urban tree problems are derived in the soil. Unfortunately, arborists and urban foresters do not have the knowledge or resources to adequately identify urban soil problems inhibiting tree growth and health. Soil quality indices have long been used in agriculture and forestry to assess soil conditions without extensive laboratory testing. Given the strong linkage between urban soil quality and tree health and growth, an urban site index would have substantial application in arboriculture and urban forestry. An urban site index could be used to determine baseline soil conditions impacts trees and also evaluate the effectiveness of soil management actions for urban trees. Furthermore, an urban site index would be an essential tool to employ towards the goal of diversification in master planting design by better matching species tolerances with site conditions. Over the past six years, we have created, refined and tested an urban site index (USI) for trees. The USI model is a field-based scoring system that can be utilized to assess and score site and soil conditions impacting urban tree health and growth. Our results from USI model testing in Chicago, IL, Toledo, OH, Cleveland, OH, Springfield, MA, Boston, MA, New York, NY and Ithaca, NY are very promising. The USI model is practical and accurate for prediction of urban tree health and growth. This presentation will cover the USI model development, our research findings on its accuracy and how the USI can be employed by arborists and urban foresters to assess urban sites for trees.