Extremely thermophilic microorganisms ( T opt ≥ 70°C) are found in geographically diverse marine and terrestrial environments and represent a wide range of growth physiologies. Extreme thermophiles thrive at high temperatures and, as such, different approaches must be taken to cultivate them in laboratory settings. Genome sequences of many extreme thermophiles have been completed and offer a glimpse into the basis for their high temperature life styles. A number of biotechnological applications have been envisioned that take strategic advantage of their thermophilicity, including the production of biohydrogen and recovery of base and precious metals from ores. As genetic systems are developed and implemented for extreme thermophiles, metabolic engineering approaches will be possible to tune the unique characteristics of these microorganisms for bioprocessing uses.