Bio-char (biomass-derived black carbon) is not only a ubiquitous form of organic carbon in soils, but also emerges as an opportunity for carbon sequestration in terrestrial ecosystems stimulated by research on Amazonian Dark Earths. It has long been known that bio-char persists in soil for centuries to millennia as it is commonly used for dating artifacts and anthropogenic deposits. More recent studies identified bio-char as the oldest fraction of organic matter in soil, confirming its recalcitrance to decomposition and mineralization. However, field and incubation studies testing the stability of freshly produced bio-char from vegetation burning did not fully corroborate the notion that bio-char is very stable in soils. Some fraction of newly produced bio-char appeared to be more labile, such as aliphatic carbon in comparison to the aromatic carbon that makes up the bulk of bio-char. Moreover, bio-char produced from different types of biomass differed greatly in its carbon forms. The conditions under which biomass is converted into bio-char had a profound impact on the carbon chemistry of the bio-char, as well. The temperature of carbon conversion is critical for the stability, with recent results indicating that temperatures of 200°C and above significantly increases stability against microbial decomposition, whereas bio-char produced at a temperature of 1000°C showed less recalcitrance to oxidation by ozone than that produced at 400°C. Apart from the chemical stability of the original material, stabilization in soil confers significant recalcitrance to bio-char. Similar to proposed mechanisms for plant litter, aggregation and formation of organo-mineral complexes indicated protection of bio-char. Whether aggregation is enhanced by bio-char is a hypothesis that requires further studies. On the other hand, priming through co-metabolism of added glucose has been observed to increase mineralization of bio-char. Although the controlling factors are still poorly quantified, bio-char is very stable in soils, which has important implications for the natural carbon cycles as well as for purposeful soil management using bio-char applications.