Forests unfold an exceptionally large ecosystem volume and expose a vast biotic surface, providing crucial ecosystem functions and services, including carbon sequestration and regional climate regulation. However, there is only little insight into the role of tree diversity in forest functioning. Hence, currently we cannot assess the consequences of species loss under global change for forest functioning. Here we review recent studies on tree diversity and ecosystem functioning in forests. Although several studies confirm the positive relationship between tree diversity and functions related to productivity, communities of biota, and soil parameters, many studies find stronger effects of species identity than diversity. We discuss the methodological shortcomings of the present study designs, including an isolated view on specific functions and the general negligence of confounding factors, and conclude that future studies can profit from exploiting information gained at the scale of tree individuals.