Fundamental problems in investigating on extinct or extant life on Mars, concern the presence of water and finding out to what extent living organisms may survive at its very low temperatures and atmosphere conditions. The collected data transmitted to Earth from the above-mentioned missions and the fascinating images of Mars, allowed a detailed reconstruction of the Red Planet surface; conjunctures were made about water as not an unknown element, at least in the past, on the planet. Scientists have now to ascertain if the astronomic and meteorological data about the actual conditions of Mars are compatible with the concept of life that we have on Earth (McKay et al., 1996). Once established that on Mars the actual conditions of water, temperature and atmosphere might be compatible with extreme life forms, which earthly organisms could colonize the Red Planet? About that, many other questions arise, such as life ever existed on Mars in the past or if some of the pre-existing organisms are still present somewhere; if terrestrial organisms could adapt to Mars conditions (Friedmann, 1986; Friedmann and Ocampo-Friedmann, 1995; Beaty et al., 2005).