This first CeOS_SE train-the-trainer workshop was an opportunity for project partners to join together to learn about the fundamentals of Citizen Science, discover best practices from across Europe and beyond and pose their questions to experts in the field. The organisers were project partners, UNITO (University of Turin). The presenters in order of appearance were: Andrea Sforzi: What is Citizen Science (and what is it not)? Alessia Smaniotto: Citizen Science in the Social Science and the Humanities Maria Cassella: Citizen Science and libraries - new opportunities of engagement with and for communities Discover more about CeOS_SE on our project website: https://ceosse-project.eu/