Global warming affects the fundamental variations of the hydrological regime and water resources. According to the climate change scenarios, the increase of air temperature as well as changes of precipitation and total evaporation might influence the annual distribution of river runoff and cause changes of the extreme runoff elements. These prognosticated changes may affect the development of water ecosystems by the new environmental conditions. The use of renewable energy sources in Lithuania as well as in EU countries is very important. One of the most usable sources of renewable energy is the river hydropower resources, which directly depend on the river runoff parameters. Following the global warming, changing river water resources may directly affect the hydropower production during the extreme nature conditions. The main aim of this book is to evaluate the past and future changes of Lithuanian rivers runoff and their consistent patterns during the XXI century using the accumulative data basis of meteorological and hydrological observation, modern climate change models, statistical methods and hydrological modeling.