The In-service Aircraft for a Global Observing System (IAGOS) is a Research Infrastructure that operates a global-scale monitoring system for atmospheric trace gases, aerosols and clouds by using the existing provisions of the global air transport system to provide essential data on climate change and air quality. It complements the global observing system in addition to ground-based networks, dedicated research campaigns and observations from satellites, balloons, and ships. IAGOS builds on the scientific and technological experience gained within the research projects MOZAIC – Measurement of Ozone and Water Vapour on Airbus in-service Aircraft – and CARIBIC – Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container. The technical, organisational and legal concept for IAGOS has been developed during its Preparatory Phase. The scientific mission has been defined as to provide high quality data throughout the troposphere and lower stratosphere, and scientific expertise to understand the evolution of atmospheric composition, air quality, and climate. The research infrastructure is distributed, and by virtue of the fact that we use commercial aircraft as a platform, it is global in nature, but does not have physical access sites. In order to provide data that is as global as possible we have airline partners from Europe, Taiwan, Hong Kong, the United States and Canada. By working with a wide range of partners, we can build up a global network of routes to provide data at cruise altitude and at more than 300 worldwide airports spanning all continents except Antarctica. The backbone of IAGOS is the 30 year time-series of water vapour and ozone in the North Atlantic flight corridor at cruise altitude and the 30 year time-series of profiles of water vapour and ozone in Frankfurt. This can now be considered as a “climate” dataset according to the WMO definition. To enrich this essential ‘climate’ dataset and provide further scientific understanding we measure the principal precursors of ozone, greenhouse gases, aerosols and clouds. The data follow FAIR principles, and are accessible though the data centre. In addition to the measured parameters, we provide value-added products such as dynamical data, and source-receptor links for tracing the provenance of air masses. Visualisation tools have been developed to support this service. The profile data collected during landing and take-off are of great interest for air quality studies and used by the Copernicus Atmosphere Monitoring Service (CAMS) for the validation of global and regional air-quality models.