Citizen Science is a rapidly expanding and diversifying field of innovation with significant implications for, and potential benefits to, society, policy, and various academic research areas. The ambition of EU-Citizen.Science is to build, fill, and promote a sustainable platform and mutual learning space for providing different tools, best practice examples and relevant scientific outcomes that are collected, curated, and made accessible to different stakeholders, ranging from interested citizens over scientific institutions up to politicians and public media in order to mainstream Citizen Science in Europe.