This book describes studies about how climate affects thermal develop­ment in lakes. Numerical models based on physics and mathematical methods were applied to lakes with different sizes, shapes and locations. Also diurnal changes and the shape of the vertical temperature profile were studied. The depth of the lake affects together with other morpho­logical features (e.g. the shape and the size) the one-dimensional basic connection between air and water, more in smaller lakes. Also the surrounding landscape affects the conditions over the lake. A special study area was southern Finland, where numerous lakes are found. Data over about 100 past years were used for lakes in the area with the radius of about 100 km. Future changes were analyzed with climate change scenarios. Models and data were compared globally. Thermal conditions in lakes are important for many areas. They influences how lakes develop, because physical conditions, e.g. existence of ice cover, create basic settings for several processes in lakes. They have been used to distinguish between lakes in different areas, as basis for observation systems and even for climate indicators, like ice cover.