European dairy production systems (DPS) face many challenges across the three pillars (3P) of sustainability. European regions are highly diverse and complex, and region-specific concepts for DPS are required. MilKey develops a whole system concept for 3P sustainability of DPS targeted to key European regions. GHG mitigation is a core aim accompanied by other environmental impacts, and analysis of economic and social factors. MilKey applies a multi-actor approach throughout the whole project. Finally, a platform for sustainable 3P concepts for DPS in key European regions and an online barn climate and emission control tool will be available. The MilKey platform will be a long-lasting multi-actor knowledge hub to increase understanding and knowledge of sustainable DPS and to intensify the dialogue between science, farmers, stakeholders and policy, also beyond the project lifetime. MilKey integrates a decision support system to assist making knowledgeable decisions on 3P sustainability.