Data for solar forecasting exist in three complementary forms: (1) ground-based measurements, (2) satellite retrievals, and (3) numerical weather prediction output. This chapter comprehensively reviews the publicly available data that can benefit solar forecasting. Starting from ground-based measurements, high-quality research-grade monitoring networks, which often serve as the reference to gauge forecast accuracy, are enumerated. Next, major sources of satellite-derived irradiance data are listed, and their properties explained. Lastly, two state-of-the-art numerical weather prediction models, one from the United States and the other Europe are thoroughly described. The reader can expect a tutorial-like guide on how these data can be acquired and of service.