Publisher Summary Biomass is the organic matter that can be converted to useful energy forms such as heat or liquid fuels. Biomass energy, the chemical energy stored in organic plant matter, is derived from solar power via photosynthesis. Biomass can be only a partial answer to the global energy need because of two fundamental constraints: its high water requirements and the inherent low photosynthetic efficiency of converting solar energy into the chemical energy of plant matter. High water requirements constrain biomass production mainly to regions where rainfall is adequate to support commercial yields, whereas the low photosynthetic efficiency can lead to land-use competition with food production. The combustion of biomass or biofuels produces air pollutants, including carbon monoxide, nitrogen oxides, and particulates such as soot and ash. The amount of pollution emitted per unit of energy generated varies widely by technology, with wood-burning stoves and fireplaces generally the worst offenders.