Biochar persistence depends on both biochar properties and environmental conditions. This chapter discusses the mechanisms by which biochar is mineralized, and how it can be predicted. Mineralization of biochars can be related to either their production conditions or their properties. Pyrolyzing organic matter decreases its mineralization by at least one and a half orders of magnitude under otherwise identical environmental conditions. The chapter discusses and provides methods to calculate long-term persistence from existing field and laboratory experiments and outlines their opportunities and constraints.