Climate change is for a large part governed by atmospheric processes, in particular the interaction between radiation and atmospheric components (e.g. aerosols, clouds, greenhouse and trace gases). Some of these components are also those with adverse health effects influencing air quality. Strengthening the ground-based component of the Earth Observing System for these key atmospheric variables has unambiguously been asserted in the IPCC Fourth Assessment Report and Thematic Strategy on air pollution of the EU. However, a coordinated research infrastructure for these observations is presently lacking. ACTRIS (Aerosols, Clouds and Trace gases Research InfraStructure Network) aims to fill this observational gap through the coordination of European ground-based network of stations equipped with advanced atmospheric probing instrumentation for aerosols, clouds and short-lived trace gases.<br/>ACTRIS is a coordinated network that contributes to: providing long-term observational data relevant to climate and air quality research produced with standardized or comparable procedures; supporting transnational access to large infrastructures strengthening collaboration in and outside the EU and access to high quality information and services to the user communities; developing new integration tools to fully exploit the use of atmospheric techniques at ground-based stations, in particular for the calibration/validation/integration of satellite sensors and for the improvement of global and regional-scale climate and air quality models. ACTRIS supports training of new users in particular young scientists in the field of atmospheric observations and promotes the development of new technologies for atmospheric observation of aerosols, clouds and trace gases through close partnership with SMEs.<br/>ACTRIS will have the essential role to support integrated research actions in Europe for building the scientific knowledge required to support policy issues on air quality and climate change.