The synthesis report gives insights into the findings, learnings, and policy implications from the SustAIn project regarding a more sustainable development of artificial intelligence (AI) systems. Within the recent debate on AI and sustainability, it is becoming increasingly clear that AI systems create significant risks for society and our planet. As part of the SustAIn project, pioneering work has been undertaken in compiling comprehensive indicators that can be used to assess the social, environmental, and economic sustainability of AI systems. The project has shown that, besides organisations that develop and implement AI, a policy approach is needed to foster more sustainable AI systems. With this report, we want to share the most important insights and results of our work and facilitate and structure the discourse on the sustainability of AI along the AI lifecycle.