This paper reviews observation- and model-based studies of stable isotopes in precipitation and discusses the Global Network for Isotopes in Precipitation (GNIP) and other regional networks and projects involving isotope observations. Using data from the latest GNIP/Isotope Hydrology Information System (ISOHIS) database, the annual and monthly variabilities of δD-δ¹⁸O relationships, the temperature effect, and the amount effect for δ¹⁸O were calculated as averages for the Northern Hemisphere, Southern Hemisphere, and globe. At the global scale, the global Meteoric Water Line (MWL) was δD = 8.00 × δ¹⁸O + 9.62 (R² = 0.99), the temperature effect was 0.53‰/°C (R² = 0.71), and the precipitation amount effect was -1.05‰/100 mm (R² = 0.20). Comparisons of modeled and observed temperature and amount effects indicate that most General Circulation Models (GCMs) have reproduced the present climate well. This paper also summarizes the use of stable isotopes in precipitation as proxy data for climate variability and water origins, as well as the results of recent isotope-observation studies, especially in the Asian monsoon regions.