Life on Earth is limited by physical and chemical extremes that define the 'habitable space' within which it operates. Aside from its requirement for liquid water, no definite limits have been established for life under any extreme. Here, we employ growth data published for 67 prokaryotic strains to explore the limitations for microbial life under combined extremes of temperature, pH, salt (NaCl) concentrations, and pressure. Our review reveals a fundamental lack of information on the tolerance of microorganisms to multiple extremes that impedes several areas of science, ranging from environmental and industrial microbiology to the search for extraterrestrial life.