Microorganisms capable of withstanding the harshest conditions of Earth are described as extremophiles: they include organisms adapted to grow and thrive in high pressure, radiation, and pH, salinity, and temperatures far above or below ambient. These microbial forms include thermophiles, psychrophiles, halophiles, acidophiles, alkaliphiles, and piezophiles, which are all adapted to their environment in a specialized physiological, biochemical, and genetic part. Due to their uniqueness in having unique metabolic pathways and biomass, such as extremozymes, they possess immense biotechnological potential, as that was derived from pharmaceuticals, biofuels, food processing, or environmental bioremediation. Extremophiles are also models to understand the origins and evolution of life and elucidate early Earth conditions and the search for extraterrestrial life in astrobiology. Further expansion of the exploration and engineering of extremophiles for industrial and scientific innovations is enabled by advances in omics technologies and synthetic biology. We review the diversity of extremophiles and their adaptive mechanisms in addition to emphasizing their applications, specifically because they enable the biotechnology and space exploration frontiers to be reached. This knowledge expands our understanding of life’s resilience and simultaneously opens doors to groundbreaking work in numerous related scientific areas.