Lake ecosystems are known to be valid sentinels for current climate changes and anthropogenic pressure because they provide indicators of these impacts either directly or indirectly through the influence of climate and human activity on their catchments. Among these indicators, to name just a few, are water temperature, dissolved organic carbon, nutrients and metals, phyto- and zooplankton composition as well as population and biodiversity of crustacea, mollusks and fish. The advantages of using lakes as tracers of climatic changes and anthropogenic impacts on aquatic ecosystems are multiple. Lake ecosystems are well constrained, confined and are studied in a sustained fashion; lakes respond directly to climate change and local and global pollution via incorporating the effects of these impacts occurring within the catchment; lakes integrate responses over time, and thus allow to avoid the random or unique single-time effects. Finally lakes of various sizes are distributed worldwide and, as such, can act as sentinels across various climatic conditions while exhibiting different degree of vulnerability to external pressure depending on their size and specific location capturing different aspects of climate change (e.g., changing precipitation regime, heat waves, permafrost thaw, invasion of new species, local and global (dispersed) pollution). However, the majority of published studies on lakes in the boreal and subarctic zone deal with Western and Northern Europe and Northern America, with quite limited information on lakes in the NW Russia. This book is intended to partially filling this gap by presenting 13 chapters describing the hydrology, hydrochemistry and hydrobiology of various lakes located in the NW European Russia, from the Finland border in the west to the Ural Mountains in the East. The thirteen chapters of the book, written by the experts in the field of biogeochemistry, limnology and zoology cover full limnetic ecosystems, from lake physical characteristics to lake water chemistry, microbiology, phytoplankton and zooplankton population, Crustacea, mollusks and fish. A multidisciplinary approach across wide geographical zones, comprising both small and large lakes of the Russian Subarctic, presented in this book, will be interesting for a large community of scholars, students, and researchers from academic and private organizations.