The chemical energy stored in biological resources can be converted into useful energy services such as heat, power, and transportation fuels. This article presents definitions such as energy crop, by-product and waste, and classifies biological materials according to their composition in four groups: lignocellulosic biomass, sugar and starches, oil biomass, and high-moisture biomass. Common primary and secondary conversion technologies for those groups are also briefly discussed. Biomass is seen as the renewable energy source with largest potential, but environmental and socio-economical impacts of bioenergy systems should be accurately evaluated in order to guarantee sustainable systems.