Technical Abstract: Conversion of biomass to biochar followed by application of the biochar to the soil increases the residence time of carbon (C) in the soil relative to application of the same biomass directly to the soil, and therefore can be considered over particular timescales to result in a net withdrawal of atmospheric carbon dioxide (CO2). The stability of biochar in soil environments is of fundamental importance because stability determines how long C applied to soil as biochar will remain sequestered. In addition, biochar stability influences how long and perhaps how effective biochar is for reducing emissions from soil of other greenhouse gases such nitrous oxide and methane. Biochar stability also determines how long biochar can provide benefits to soil and water quality. The long-term benefits of biochar additions to soil on soil and water quality may include: improved nutrient retention and nutrient availability, a liming effect, reduced leaching of nutrients and other contaminants, increased water availability to plants, improved mycorrhizal activity and possibly benefits to other groups of microorganisms and their function in soil. If biochar decomposes rapidly, these aforementioned benefits would be affected in extent and duration. This chapter explores the extent of biochar stability in soils, the mechanisms controlling its decay and stability, and implications of physical export for biochar stability.