The harshness of the environment at its surface severely limits the survival of unprotected terrestrial organisms on Mars. However, the Martian geologic record is consistent with an earlier history with much different climatic conditions, more conducive to life, followed by an extended climate change which led to the currently inhospitable state. Such a transition is ideal for evolutionary adaptation to new conditions because it provides both the forcing function and the extensive time for progressive specializations. It is shown that against each of the current limits to life (organics deprivation; restricted amounts and physical forms of H2O; limited availability of other key nutrients; ionizing radiation insult; and strongly oxidizing chemical species), there are plausible countermeasure solutions. Most, but not all, of these mechanisms already exist as known examples in terrestrial biota. A combination of these adaptations could conceivably allow highly tailored organisms, either active or dormant, to survive at or very near the surface of Mars. Even in carefully preserved regolith samples returned to Earth, microorganisms could be difficult to detect without specific biochemical probes and/or purposeful biological amplification by selective enhancement of the supply of nutrients, optimized incubation parameters, and highly sensitive detection of metabolic activity. ¿ 1998 American Geophysical Union