We will expand our horizons in biology by learning about some of the most extreme microorganisms on the planet – microorganisms that thrive without oxygen in deep marine sediments and in the Earth’s crust, under high temperatures in boiling hot springs or in superheated deep-sea water under high pressure, and under chemical stress factors (high sulfide and heavy metal concentrations) that were once thought to be incompatible with life. Numerous extremophilic (extreme-loving) microorganisms of different metabolic types have been isolated in the laboratory as pure cultures; others have been observed in Nature but have so far resisted cultivation. Extremophiles provide opportunities to study the unusual and strange biochemistry that allows them to thrive in their unique habitats; they are also valuable model systems for potential life on other planets. We will get to know the unusual habitats where extremophiles are found, for example hot springs and volcanic areas on land (Yellowstone) and in the ocean (hydrothermal vents), and we will explore the earliest history of extremophiles as some of the most ancient microorganisms on Earth.