This article reviews current knowledge on microbial communities inhabiting endolithic habitats in the arid and hyper-arid regions of our planet. In these extremely dry environments, the most common survival strategy is to colonize the interiors of rocks. This habitat provides thermal buffering, physical stability, and protection against incident UV radiation, excessive photosynthetically active radiation, and freeze-thaw events. Above all, through water retention in the rocks' network of pores and fissures, moisture is made available. Some authors have argued that dry environments pose the most extreme set of conditions faced by microorganisms. Microbial cells need to withstand the biochemical stresses created by the lack of water, along with temperature fluctuations and/or high salinity. In this review, we also address the variety of ways in which microorganisms deal with the lack of moisture in hyper-arid environments and point out the diversity of microorganisms that are able to cope with only the scarcest presence of water. Finally, we discuss the important clues to the history of life on Earth, and perhaps other places in our solar system, that have emerged from the study of extreme microbial ecosystems.