Hydrobiological research, or more precisely hydroecological research, is a demanding undertaking. Its difficulty lies in the fact that it involves fieldwork requiring boats, support personnel, instruments, and various types of equipment. A hydrobiologist must be capable of conducting meticulous laboratory work while also being willing, in any season and under any weather conditions, to visit water bodies and engage in the physically demanding task of collecting samples. Yet the challenges of hydrobiological research are not limited to, nor even primarily defined by, these factors. The study of such complex natural systems as aquatic ecosystems cannot be accomplished by a single individual. It can only be carried out by a team of specialists from diverse disciplines, including even non-biological fields such as hydrochemistry. Organizing and coordinating the efforts and interests of all participants is often difficult. When research must be maintained over many years, the complexity of the situation and the associated difficulties increase many times over. It is therefore not surprising that examples of long-term and comprehensive biolimnological investigations of specific water bodies are so rare. In recent years, however, such studies have become especially important because only through long-term observations and detailed biological investigations can one distinguish the consequences of human impact from changes caused by hydrometeorological and other physical-geographical factors. Only in this way can we understand how water quality and the productivity of aquatic ecosystems depend on anthropogenic influences, thereby addressing the central problem of modern hydroecology. The collective monograph The Ecosystem of the Naroch Lake Group presents the results of many years of research conducted at the Naroch Biological Station, which was established in 1946 on the initiative of the renowned zoologist and hydrobiologist Lev Aleksandrovich Zenkevich. The investigations focused on Lake Naroch and its connected lakes, Myastro and Batorino. These three lakes are arranged in the order of water flow as Lake Batorino, Lake Myastro, and Lake Naroch, and they share a common drainage basin, which allows us to consider them together as the ecosystem of the Naroch lake group. Lake Naroch occupies a special place among the water bodies of the Byelorussian Soviet Socialist Republic. Its abundant natural resources and its considerable recreational, health-resort, and aesthetic value make its protection and rational utilization a matter of national importance. Each year the lake experiences increasing anthropogenic pressure, primarily from recreational activities and tourism. At present, signs of the onset of eutrophication have already appeared in the lake, and in the long term this may lead to significant deterioration in water quality. Consequently, it has become especially important to provide a scientific basis for environmental measures aimed at maintaining the lake ecosystem in a favorable condition, to determine the rate of eutrophication, and to forecast possible changes in productivity and water quality. Systematic long-term studies of Lake Naroch are also of great significance for general aquatic biology because they help to clarify the common characteristics of lakes of this type, with Lake Naroch serving as a model water body. Lake Naroch belongs to the mesotrophic lakes that are widely distributed throughout the Soviet Union. Equally important is the fact that the neighboring lakes Myastro and Batorino differ markedly from Lake Naroch in typological terms and may serve as excellent model water bodies for eutrophic and highly eutrophic lakes, respectively. Year-round ecological monitoring of Lakes Naroch, Myastro, and Batorino, including observations of water temperature and transparency, major hydrochemical and microbiological indicators, chlorophyll content of phytoplankton, and the composition, abundance, and biomass of phytoplankton, zooplankton, and benthic organisms, has been incorporated into the international Man and the Biosphere research program as Project No. 5, Ecological Effects, Use, and Resources of Continental Waters under Human Influence. This monograph describes the hydrobiological investigations conducted in Lakes Naroch, Myastro, and Batorino, the objectives pursued during different stages of research, and the principal results obtained. These achievements received broad recognition within the Soviet Union and gained international renown because of their important contribution to the implementation of the International Biological Programme. At the same time, the body of work assembled by different authors clearly reveals existing gaps in knowledge, as well as inconsistencies among the various chapters of the book in terms of format and level of presentation. Naturally, such shortcomings could only be remedied to a limited extent during the editorial process. Nevertheless, it is both important and necessary to have a clear understanding of what has already been accomplished and what remains to be done, so that these considerations may be taken into account when planning future investigations. Accordingly, the significance and value of this book lie not only in its synthesis of many years of work by Belarusian hydrobiologists, but also in its potential to improve the effectiveness of future studies. Such studies are aimed both at elucidating the general biological principles governing the functioning of aquatic ecosystems and at addressing the practical problems of protecting and rationally utilizing the Naroch lake group.