Through our consumption behaviour we, individuals, are responsible for 72 per cent of the global greenhouse gas emissions. These are created by the way we live, travel, and eat and by what we buy. So far, the attempts to change people's habits by appealing to their rationale have not produced significant behaviour change. Aiming to help close the action gap between climate awareness and individual action, and to increase citizen participation in sustainability topics, the project builds a data-driven momentum for sustainable behaviour change across eight European countries. It does this by engaging citizens through a digital application to co-research, co-develop and uptake everyday life solutions for climate change, providing tools for the collection, monitoring and analysis of their environmental and consumption data. In the application, the citizens find out how their lifestyle and habits impact the environment. They will also see a list of smart everyday actions that are relevant to reduce their environmental impact. The application enables citizens to actively create data while monitoring their lifestyle induced climate impact and behaviour. The collected data will be used to improve the application itself, and, importantly to empower citizens to gain agency in research and policy-making. The ambition is to engage a total of four million EU citizens. The figure is based on the previous national success with a web-based carbon emission calculator, which is innovated further with the use of citizen science, by co-research at living labs, co-creating personalized sustainability plans, as well as with the integration of behaviour science knowledge into the co-research and design process. The overarching goal of PSLifeStyle is to co-create a web- and mobile-based sustainable lifestyle tool and open platform, that will enable, empower and encourage European citizens to take their personalized steps towards more sustainable lifestyles.