The application of biochar for sequestering carbon in soils has attracted recent attention due to both its prospect for mitigating climate change and its ability to act as an organic substitute for organic fertilizers. In order to determine the effectiveness of the addition of biochar to soils as a climate change mitigation strategy, it is imperative that factors relating to the long-term stability of biochar in soils be understood. A variety of different methods have been utilized to assess biochar stability. Some methods study degradation of biochar in soils, such as the use of field and laboratory incubations, while others assess certain biochar properties related to its stability, such as aromaticity. Through the compilation and assessment of results obtained via a variety of analytical methods, a number of factors related to pyrolysis and soil conditions that impact upon the stability of biochar in soils may be identified. Pyrolysis temperatures ranging between 600oC and 700oC have been found to be most effective in increasing biochar stability by lowering H:C and O:C ratios, increasing aromaticity and degrees of aromatic condensation, promoting negative or less positive soil priming effects, and decreasing carbon mineralization. Furthermore, biochars produced from feedstocks with higher lignin contents and lower cellulose and ash contents are more likely to possess properties that promote stability. Studies suggest that biochar may be more stable in mineral rich soils of neutral to alkaline pH, low soil organic content, and in temperatures of less than 20oC.