The purpose of this monograph is to bring together under one cover results of research on phenomena drawn from the fields of chemistry, biochemistry, bio­ physics, virology,and cell biology. The processes and reactions considered have one important feature in common: they are endothermic and, therefore, entropy­ driven. They are, in the main, reversible reactions leading to the formation of large structures, some of which play critical roles in life processes. If one thinks only of the subunits and of the structures they form upon poly­ merization, it seems to be a contradiction that such reactions can be driven by an increase in entropy; entropy is a measure of disorder. The increase in entropy must come from some other source, usually from the release of something coincidental to polymerization. That something has been shown to be water for the case of the polymerization of tobacco mosaic virus protein. Because of the remarkable similarity of the other processes to this one, it is a permissable inference that the release of water is the source of the entropy increase and therefore the driving force for all of them. The reactions and processes brought together in this book are still the sub­ jects of active research. ;~ny of the detailed interpretations presented here must be regarded as tentative, subject to modification as new information becomes available. However, the main characteristic of each reaction or pro­ cess, its endothermic or entropy-driven nature, is well established in all but one or two instances.