Modern methods and approaches, such as the analysis of molecular sequences to infer evolutionary relationships among organisms, have provided vast new sets of data to further our understanding ofliving organisms, but there remain enigmas in the biological world that will keep scientists working and thinking for decades. Microorganisms by virtue of their small size and almost unbounded diversity provide ample examples of intriguing mysteries that are being challenged with all of the techniques the modern scientific arsenal can provide. One whole arena of this battle to resolve puzzling mysteries about various microorganisms is the almost unbelievable ability of many micro-organisms to live in extreme environments. Whether the challenge is extreme heat, cold, pressure, hyper­ salinity, alkalinity or acidity, some micro-organisms live now where no life might seem possible. This fascinating state of affairs is the context for this present volume edited by Joseph Seckbach. This Volume is a compilation of many of the especially interesting questions and biological challenges that arise in the consideration of microorganisms in general and the extremophiles in particular.