The project of saproxylic insects living in hollow trees lies in the field of urban ecology. It concerns the conservation value of hollow trees and urban areas, and concentrates on the ecology of saproxylic beetles and other invertebrates living in hollow trees. Hollow trees are very important for the biodiversity. Many species, of which some are rare or endangered, are living or dependent on tree cavities. Nowadays hollow trees can be found mainly in urban and suburban areas. Because most of the specialized saproxylic beetles dependent on hollow trees also favor warm and light conditions e.g. managed parks, it makes the urban areas vitally important for the species and their conservation. The project cooperates with Finnish Forest Research Institute and the City of Helsinki. It started with examining the hollow trees as a habitat for saproxylic invertebrates and continued to study the tree and surrounding level characteristics affecting the saproxylic beetles assemblage living in tree cavities. The aim is to find the most important trees for the diversity of the saproxylic species and to share information on hollow trees and their biodiversity in urban areas for the conservation efforts. In the future, the work can continue e.g. on the modeling of metapopulation dynamics of saproxylic beetles living in hollow trees and in practice creating and maintaining the continuity of hollow bearing trees in urban areas.