This paper provides a broad overview of the use of plant material as a source of usable energy. The problem of gathering and interpreting data on the use of biomass is discussed. From the limited data available it is inferred that bio energy contributes about 15% of the global energy budget. The paper deals with a variety of other major topics: the role of biomass in the energy system, differences in that role in industrialized and developing countries, conversion technologies, environmental considerations and the multiple uses of biomass (food for people, feed for animals and fibre for construction material and other uses as well as energy sources). The ability of the biosphere to provide adequate amounts of primary energy is limited, especially if forecasts of population growth, and associated increases in the demand for food, feed and fibre, turn out to be accurate. By improving the efficiency ofbioenergy use, which is currently very low, it should be possible to deliver more tertiary energy for the same primary energy input.