The International Reference Ionosphere (IRI) is an international project jointly sponsored by the Committee on Space Research (COSPAR) and the International Union of Radioscience (URSI) to develop and improve a reference model for the most important plasma parameters in the Earth's ionosphere. The model is primarily based on experimental evidence using all available ground and space data sources and should not depend on evolving theoretical understanding of ionospheric processes. The IRI model describes monthly averages of electron density, electron temperature, ion temperature, and ion composition. It represents variations with altitude, latitude, longitude, date, and time of day. External drivers include solar indices, ionospheric index, and magnetic indices. Additional output parameters include vertical ionospheric total electron content, ion drift at the equator, and occurrence probability for spread-F and F1 layer. The IRI model is widely used and validated, recognized as a standard by COSPAR, URSI, ITU, ECCS, and ISO. It includes effects not yet discovered or explained by theory. The IRI team's global distribution guarantees access to the global database. New models are easily phased in with validation help from users. The IRI model has been improved with new data sources and methods, including data from digisonde stations and radio-occultation data. The IRI model is used for space weather applications, GNSS data, and real-time ionosphere representation. IRI workshops and publications have been held globally, and the model is used in various scientific journals. The IRI model is continuously improved with new data and methods, and it is used for ionospheric weather predictions and space research.The International Reference Ionosphere (IRI) is an international project jointly sponsored by the Committee on Space Research (COSPAR) and the International Union of Radioscience (URSI) to develop and improve a reference model for the most important plasma parameters in the Earth's ionosphere. The model is primarily based on experimental evidence using all available ground and space data sources and should not depend on evolving theoretical understanding of ionospheric processes. The IRI model describes monthly averages of electron density, electron temperature, ion temperature, and ion composition. It represents variations with altitude, latitude, longitude, date, and time of day. External drivers include solar indices, ionospheric index, and magnetic indices. Additional output parameters include vertical ionospheric total electron content, ion drift at the equator, and occurrence probability for spread-F and F1 layer. The IRI model is widely used and validated, recognized as a standard by COSPAR, URSI, ITU, ECCS, and ISO. It includes effects not yet discovered or explained by theory. The IRI team's global distribution guarantees access to the global database. New models are easily phased in with validation help from users. The IRI model has been improved with new data sources and methods, including data from digisonde stations and radio-occultation data. The IRI model is used for space weather applications, GNSS data, and real-time ionosphere representation. IRI workshops and publications have been held globally, and the model is used in various scientific journals. The IRI model is continuously improved with new data and methods, and it is used for ionospheric weather predictions and space research.