A diverse group of anoxygenic phototrophic bacteria thrive in habitats characterized by extremes of temperature, pH, or salinity. These ‘extremophilic’ anoxygenic phototrophs are optimally adapted to the conditions of their habitats and are ideal model systems for defining the physiochemical limits of photosynthesis. Extremophilic phototrophs have provided new insight into the evolution of photosynthesis and play ecological roles as primary producers in their unusual habitats.