This chapter reviews data sources and their integration in hydrological models. It deals with available data sources in hydrometeorology and their usage. Hydrometeorological states include land-surface temperature, near-surface soil moisture, vegetation cover, snow cover, snow-water equivalent, water quality, and landscape roughness. Out of the many available Earth observation (EO) hydrometeorological data products, five products with high potential benefit for hydrological modelling have been used in this research: land-use/land cover, leaf area index, actual evaporation, soil moisture, and precipitation. Integration of in-situ monitoring data with EO data also has been studied by many researchers. Numerical Weather Prediction models integrate in-situ and EO data mainly through data assimilation. In addition, EO also can provide more general data such as land-use and soil erosion monitoring. Generally, EO information is used in semi-spatially distributed and fully spatially distributed models. Many researchers are working on assimilation of EO data with land surface models to improve accuracy of hydrological model simulations.