The EU-H2020 project ACACIA runs for 3.5 years from January 2020 to June 2023. Non-CO2 emissions of aviation may impact climate as much as aviation's carbon dioxide (CO2) emissions do. However, the impact of the non-CO2 effects (e.g. ozone and methane from NOx emissions, contrails, indirect aerosol effects) is associated with much larger uncertainties. Some of these effects might result in a relatively large cooling. ACACIA has four aims for scientifically based and internationally harmonised policies and regulations for a more climate-friendly aviation system.    We will improve scientific understanding of those impacts that have the largest uncertainty, in particular, the indirect effect of aviation soot and aerosol on clouds.    We will identify needs for international measurement campaigns to constrain our numerical models and theories with data and we will formulate several design options for such campaigns.    Putting all aviation effects on a common scale will allow providing an updated climate impact assessment. Uncertainties will be treated in a transparent way, such that trade-offs between different mitigation strategies can be evaluated explicitly. This helps our final aim ...    ... to provide the knowledge basis and strategic guidance for future implementation of mitigation options, giving robust recommendations for no-regret strategies for achieving reduced climate impact of aviation. To this end, ACACIA brings together research across scales (from plume to global scale), from the laboratory experiments to global models, and it proceeds from fundamental physics and chemistry to the provision of recommendations for policy, regulatory bodies, and other stakeholders in the aviation business. Additionally, ACACIA will cooperate with international partners, both research institutions and organisations. In ACACIA, a total of 11 participants from 7 European countries are collaborating. ACACIA is organised in 6 work packages. Further goals are the organisation of an international conference.