Trees outside forests (ToF) is often a misunderstood category. This is the case in Brazil as shown by the lack of data on ToF reported until now by the country. In this article, ToF are understood in relation to the FAO definition of forest because it is the definition used in Brazil for the National Forest Inventory. I provide a definition of ToF, detail the main sets as inferred from this definition, propose to focus on a category of ToF as an efficient and realistic first step towards the assessment of ToF countrywide, and I illustrate the diversity of ToF-based systems in Brazil, from relatively isolated trees in agroforestry fields to dense mixed tree formations that cannot be distinguished from forests on satellite images. A recent publication has placed Brazil as the world leader in terms of the total biomass carbon stored by one ToF set, trees on agricultural land. This result could stimulate the desire for Brazil to better assess not only trees on agricultural land but also trees on urban land, the two major sets of trees outside forests. The present paper can help those who will undertake this challenging and exciting task.