This chapter discusses the resistance of spores of Bacillales and Clostridiales, with focus Bacillus species, in which spore resistance mechanisms are best understood, with most specific knowledge from work with B. subtilis spores. The spore coat plays a major role in spore resistance. First, some protective enzymes that are loosely associated with or integral components of the coat can inactivate toxic chemicals; two such enzymes are superoxide dismutase and catalase. Second, coat protein appears to act like ‘’reactive armor’’ detoxifying damaging chemicals before they can react with components in the spore’s interior. Spore structure is important in spore resistance and because spore structure is different than that of growing cells, the major features of spore structure and how these features contribute to spore resistance is outlined in the chapter. Spores of Bacillales and Clostridiales are more resistant than growing cells to stress factors and studies have attempted to correlate differences in spore resistance with differences in spore structural or biochemical properties. It is reasonable to ask whether conclusions from work on one or two species are applicable to spores of other species, available evidence indicates that basic mechanisms of spore resistance are similar in spores of all Bacillales and Clostridiales. Some resistance of core proteins to damage, in particular to chemicals, is likely because of mechanisms that also protect DNA, including detoxification of reactive chemicals by enzymes in the coat/exosporium, inactivation of toxic chemicals by reaction with coat components, and the low permeability of the inner membrane.