Many agents of physical, chemical, or biological nature, have the potential for causing cell stress. These agents are called stressors and their effects on cells are due to protein denaturation. Cells, microbes, for instance, perform their physiological functions and survive stress only if they have their proteins in the necessary concentrations and shapes. To be functional a protein shape must conform to a specific three-dimensional arrangement, named the native configuration. When a stressor (e.g., temperature elevation or heat shock, decrease in pH, hypersalinity, heavy metals) hits a microbe, it causes proteins to lose their native configuration, which is to say that stressors cause protein denaturation. The cell mounts an anti-stress response: house-keeping genes are down-regulated and stress genes are activated. Among the latter are the genes that produce the Hsp70(DnaK), Hsp60, and small heat protein (sHsp) families of stress proteins. Hsp70(DnaK) is part of the molecular chaperone machine together with Hsp40(DnaJ) and GrpE, and Hsp60 is a component of the chaperonin complex. Both the chaperone machine and the chaperonins play a crucial role in assisting microbial proteins to reach their native, functional configuration and to regain it when it is partially lost due to stress. Proteins that are denatured beyond repair are degraded by proteases so they do not accumulate and become a burden to the cell. All Archaea studied to date possess chaperonins but only some methanogens have the chaperone machine. A recent genome survey indicates that Archaea do not harbor well conserved equivalents of the co-chaperones trigger factor, Hip, Hop, BAG-1, and NAC, although the data suggest that Archaea have proteins related to Hop and to the NAC alpha subunit whose functions remain to be elucidated. Other anti-stress means involve osmolytes, ion traffic, and formation of multicellular structures. All cellular anti-stress mechanisms depend on genes whose products are directly involved in counteracting the effects of stressors, or are regulators. The latter proteins monitor and modulate gene activity. Biomethanation depends on the concerted action of at least three groups of microbes, the methanogens being one of them. Their anti-stress mechanisms are briefly discussed in this Chapter from the standpoint of their role in biomethanation with emphasis on their potential for optimizing bioreactor performance. Bioreactors usually contain stressors that come with the influent, or are produced during the digestion process. If the stressors reach levels above those that can be dealt with by the anti-stress mechanisms of the microbes in the bioreactor, the microbes will die or at least cease to function. The bioreactor will malfunction and crash. Manipulation of genes involved in the anti-stress response, particularly those pertinent to the synthesis and regulation of the Hsp70(DnaK) and Hsp60 molecular machines, is a promising avenue for improving the capacity of microbes to withstand stress, and thus to continue biomethanation even when the bioreactor is loaded with harsh waste. The engineering of methanogenic consortia with stress-resistant microbes, made on demand for efficient bioprocessing of stressor-containing effluents and wastes, is a tangible possibility for the near future. This promising biotechnological development will soon become a reality due to the advances in the study of the stress response and anti-stress mechanisms at the molecular and genetic levels.