The identification and development of new sources of energy is a major challenge for our future and will continue to appear in news reports on a regular basis. This entry provides some basic background on a particular type of alternative energy source that is beginning to receive more attention: biofuels. These are sources of energy (solid, liquid, or gas) derived from biomass, which is defined as any organic matter that is produced and available on a recurring basis. Biofuels, therefore, are considered to be renewable sources of energy. The energy is supplied by the sun through photosynthesis, which captures solar energy and combines it with atmospheric carbon dioxide to convert it to chemical energy in various plant compounds. After the plant material is harvested, converted ...