Trees within an urban community provide measurable aesthetic, social, ecological and economic benefits. When growing normally and stably, they contribute to making a city more livable and comfortable for its inhabitants. However, as large physical structures in close proximity to people and property, their failure can cause harm. The science of tree stability analysis uses both biological and engineering principles in determining a tree’s structural soundness and predicting the probability of failure. Nondestructive testing methods of locating and quantifying wood decay and defect are used to measure the physical condition of trees within the urban forest to promote public safety. These methods are of special value to the urban forest managers and arborists responsible for the general safety of city residents, roadway transportation, and utility corridors. This chapter will discuss the commonly used methods of visual inspection, acoustic testing, and microdrill resistance with two case studies presented to illustrate how they are used in combination plus how to best interpret data collected in evaluating the nature and threat of discovered defects.