For many years, the authors have investigated the adaptive role of heat shock proteins (HSPs) in different animals, including the representatives of homothermic and poikilothermic organisms that inhabit regions with contrasting thermal conditions. This book will summarize the data accumulated in the course of these studies and describe the general molecular mechanisms underlying the adaptation of various organisms to aggressive environments. We also concentrate on different evolutionary trends characteristic for HSP systems in the course of adaptation to fluctuating environmental conditions. In addition, we describe the peculiarities in the regulatory regions of heat shock genes necessary for fine tuning of these systems providing the adaptation to adverse conditions. Special emphasis is given to the role of mobile elements in the evolution and functioning of various groups of HSP genes. The book combines the results of field studies and laboratory analysis of stress genes systems.