Extremophiles are microorganisms that thrive under physical and chemical extremes that were once considered incompatible with life. This chapter outlines the physiological boundaries of life across temperature, pH, salinity, pressure, radiation, and desiccation and distinguishes between growth and tolerance. Core molecular adaptations such as protein stabilization, archaeal membranes, DNA repair, and compatible solutes have been highlighted as strategies that enable survival. We also summarize the key industrial and ecological applications, including extremozymes, bioremediation, and bioenergy production, and relate these contributions to the UN Sustainable Development Goals. Finally, we note the remaining challenges in cultivation, genetic manipulation, and scale-up and highlight future opportunities in synthetic biology and space biotechnology.