Climate change is likely to impact the water budget and the water level fluctuation of large lakes in Hungary. The basic components of the water budget, which determine the lake level, are the precipitation falling on the lake, the surface and subsurface runoff from the catchment, the outflow, and the evaporation. For Lake Balaton, Lake Fertő, and Lake Velence, relatively long and detailed data sets are available for each water budget component. In the TDK dissertation, first, the reliability of component values, then the long-term change of the water balance should be examined using the measured data. Third, the direction and the magnitude of the future change should be estimated accordingly. The research aims to provide a detailed picture of the changes in the water resources and the water balance components that have already taken place in recent decades, and to estimate their future trends in a changing climate. In addition to the changes in the long-term averages, we aim to examine and forecast the changes in the intra-annual distribution of each component, including the lake level.