The monograph Issyk-Kul and the Trends of Its Natural Evolution is dedicated to the memory of Professor Arseny Vladimirovich Shnitnikov, Doctor of Geographical Sciences, a distinguished geographer renowned for his studies of limnology and the rhythmicity of natural processes, who made a major contribution to the investigation of the natural environment of Lake Issyk-Kul. This book presents the results of research carried out by a group of scientists from the Institute of Limnology of the Academy of Sciences of the USSR under the direction of A. V. Shnitnikov. Their work in the Tien Shan and Pamir Mountains was aimed at identifying the principal trends in the contemporary natural development of Lake Issyk-Kul and other high-mountain lakes. In recent years, the continued decline of Lake Issyk-Kul's water level has given rise to extensive discussion concerning the causes of this phenomenon and the possibility of regulating the desiccation processes within the Issyk-Kul Basin. The economic development of the basin, the growth of recreation and tourism, and the increasing population of the Issyk-Kul region all underscore the need to study the area's water resources and their future prospects. Particularly important is the clarification of the relative contributions of natural and anthropogenic factors to the overall decline in lake level. An accurate assessment of this relationship largely determines the planning and development of water-management measures within the basin and ultimately affects the efficiency of investments. Consequently, identifying the principal natural factors influencing the water balance of Lake Issyk-Kul and determining the pattern of its level fluctuations constitutes one of the most important tasks of modern research. Its solution will help establish the most rational approaches to regulating the lake's hydrological regime. At present, four principal hypotheses are used to explain the modern trend of changes in the water level of Lake Issyk-Kul: the existence of significant underground outflow from the lake; the separation of the Chu River, which formerly flowed into the lake; increased withdrawals of river water within the basin for agricultural irrigation; and the influence of climatic change. Each of these hypotheses, considered individually, is capable of satisfactorily explaining the long-term persistence of the lake's negative water balance, a phenomenon observed throughout the present century. The question therefore remains: what is the decisive factor governing the pattern of lake-level variation? To date, the problem remains unresolved due to an insufficient understanding of the laws governing the natural evolution of the Tien Shan. Research on the characteristics of moisture circulation within the lake basin is still extremely limited, making it impossible to determine the quantity of water vapor carried out of the Issyk-Kul Basin by air currents or to evaluate the contribution of evaporation from the lake surface and irrigated lands to the overall water balance. Existing views concerning underground outflow from the lake vary widely, ranging from a complete denial of its existence to the assertion that it is the principal cause of the negative water balance. It can be stated with confidence that, at the current stage of research, this problem cannot yet be conclusively resolved. Nevertheless, on the basis of available hydrochemical studies, it is possible to conclude with reasonable certainty that no subsurface flow from Lake Issyk-Kul to the Chu River exists in the Boom Gorge region. There is no doubt that the Chu River flowed into the lake in the past. This is evidenced by a deep submerged valley extending eastward through Rybachy Bay of Lake Issyk-Kul. The "separation" of the river from the lake due to tectonic or hydrodynamic causes may have occurred only once. Its consequence should have been a single major decline in lake level throughout the lake's history. However, historical and archaeological evidence indicates that during the past several thousand years the lake level has repeatedly undergone substantial rises and falls relative to its present position. As for the large-scale diversion of basin rivers for irrigation, its impact has been considered significant only since the second half of the twentieth century, whereas the directed decline of Lake Issyk-Kul began approximately 150 years ago. This change in lake-level position is therefore most plausibly explained from a climatic perspective. The authors further develop A. V. Shnitnikov's concept concerning the role of climate change in determining the condition of lakes. They analyze observational data on the current state of the natural components of the Issyk-Kul Basin and their changes through time. Particular attention is paid to identifying cycles of different durations and long-term trends in climatic elements, basin humidity conditions, groundwater regimes, and fluctuations in Lake Issyk-Kul water level. An important section of the monograph is devoted to the study of the components of the lake's water and heat balance, the use of water for irrigated agriculture within the basin, and the magnitude of its influence on lake-level fluctuations. However, it would be methodologically incorrect to study and analyze trends in the development of natural conditions within the lake basin solely on the basis of a short period of instrumental observations, without considering historical changes in lake level, the condition of other water bodies in the region, and broader humid-climatic cycles. For this reason, the authors attach great importance to retrospective analyses of the dynamics of natural processes within the basin, comparing historical and archaeological information with geomorphological, palynological, biogeochemical, and other evidence concerning changes in the humid components of the Issyk-Kul Basin during the Holocene. The characteristics of the present condition and temporal changes of other high-mountain lakes in the Tien Shan and Pamir regions, considered in the monograph as indicators of climate change, contribute to the identification of general trends in the natural development of mountain environments. Overall, the natural evolutionary trends of Lake Issyk-Kul are examined within the broader context of the dynamic physical-geographical conditions of the vast region encompassing the mountain systems of Central Asia. The research was carried out in close scientific cooperation with the Department of Geography of Leningrad State Zhdanov University, the Institute of Zoology and Botany of the Academy of Sciences of Estonia, and the Tien Shan High-Mountain Physical Geography Station of the Academy of Sciences of Kyrgyzstan. The authors express their sincere gratitude to I. S. Copland-Dixon and E. V. Maksimov for their valuable comments on the manuscript; to A. A. Liiva and T. S. Linne for methodological guidance in fieldwork and assistance with radiocarbon dating of samples; and to S. U. Umurzakov, Chairman of the Kyrgyz Geographical Society, for his constructive criticism and numerous important suggestions during the preparation of the monograph.