In its simplest terms, biomass is all the plant matter found on our planet. Biomass is produced directly by photosynthesis, the fundamental engine of life on earth. Plant photosynthesis uses energy from the sun to combine carbon dioxide from the atmosphere with water to produce organic plant matter. More inclusive definitions are possible. For example, animal products and waste can be included in the definition of biomass. Animals, like plants, are renewable; but animals clearly are one step removed from the direct use of sunlight. Using animal rather than plant material thus leads to substantially less efficient use of our planet’s ultimate renewable resource, the sun. So, we emphasize plant matter in our definition of biomass. It is the photosynthetic capability of plants to utilize carbon dioxide from the atmosphere that leads to its designation as a “carbon neutral” fuel, meaning that it does not introduce new carbon into the atmosphere. In reality—as discussed later in the description of life cycle assessments of biomass use—we find that biomass fuels are not quite carbon neutral, because somewhere in the life cycle of their production, conversion, and distribution, some fossil energy carbon is released.