Biological fertilization in energy forestry includes, in its widest sense, both (a) soil amendments with various forms of organic matter, e.g. farmyard manure, sewage sludge, wastewater, contaminated ground water, and green manure (including leguminous pre-crops, plant litter and harvest residues), (b) direct cultivation of symbiotically nitrogen-fixing woody plants or willow/poplar plantations intermixed with nitrogen-fixing ley crops or trees; and (c) stimulation or introduction of other rhizosphere microorganisms that interact positively with the energy forest crop with respect to plant nutrient uptake (e.g. mycorrhiza). Certain microbial aspects on the use of organic matter and the role of root-symbionts and other plant growth stimulating rhizosphere microorganisms are dealt with. The main emphasis, however, is laid on experiences of direct cultivation of woody leguminous and actinorhizal plants for energy forest purposes under both temperate and tropical conditions. Plant selection, plantation management, rhizobiology, climatic considerations and environmental aspects are discussed.