Many influential organisations foresee biomass playing a key role in a future, more sustainable, global energy supply matrix. Countries such as Austria, Brazil, Denmark, Finland, Sweden, India, the USA and the UK are actively encouraging the use of biomass for energy, and pushing forward the development of the necessary knowledge and technology for modern biomass energy systems. There is a growing consensus that renewable energy must progressively displace the use of fossil fuels, with fears of global climate change adding urgency to this need. Among the available types of renewable energy biomass is unique in its ability to provide solid, liquid and gaseous fuels which can be stored and transported. The potential resource for bioenergy is large, especially in forest-rich nations, in richer countries where there is a surplus of agricultural land, and in many low latitude countries where high biomass yields are possible. Therefore we expect biomass to be an important fuel of the future, but this cannot be taken for granted. The systems adopted must demonstrate clear environmental and social benefits relative to alternatives if the potential is to be realised. These benefits are not inherent to biomass energy, but depend on site- and fuel cycle-specific factors. Life-cycle analysis and evaluation of external costs are important means for assessing the social and environmental pros and cons of bioenergy systems.