Trees outside forests (TOF) have recently assumed importance in view of their economic, ecological, and climatic role. As a result, several countries have extended their scope to include TOF in their national forest inventories (NFIs) and landscape inventory systems. However, a huge variability exists in its assessment approach across different countries. Moreover, TOF definition, classification, assessment techniques, and monitoring methodology are suffering lack of attention in terms of uniformity across the globe. The present work focusses on TOF resources, nomenclature, and suitability of assessment methods for monitoring TOF cover and biomass inventory. TOF occur on different land use categories, making their assessment a little complicated. This chapter presents an overview of diverse and integrated techniques of assessing TOF using field methods complimented by robust remote sensing techniques. Field inventory methods are irreplaceable owing to their importance in establishing relationships with such advanced tools. Remote sensing applications have shown promising results in deciphering TOF resources recently by adopting multiple satellite data and classification techniques. A lot of case studies conducted in the past have demonstrated that optical, RADAR, and LiDAR (light detection and ranging) remote sensing methods do offer advantages over the traditional methods in terms of accuracy, time, and cost-effectiveness. Assessment of TOF is widely being recognized as an essential topic in sustainable natural resource management due to their role in offering variety of goods such as timber, fruits, and fodder as well as services like water, carbon, biodiversity, etc. TOF offer great potential for sequestering carbon and are high in case of agroforestry systems as compared to that of the dense canopy cover. Under the recent trend of urbanization, TOF are going to be important in climate amelioration in habitations. Another aspect is the existence of huge chunk of wastelands, which could be potential sites for hill agroforestry systems under TOF in view of enhancing carbon sinks under CDM and REDD+ mechanism. Carbon inventory assessment programs at national and international levels desire for adoption of latest methods including remote sensing and geographic information system (GIS) integrated with field inventory and advanced algorithms for qualitative and quantitative assessment of TOF resources.