This publication is primarily developed to guide those interested in identifying, understanding, and enhancing policy and behavioural changes resulting from citizen science initiatives. It thus targets those initiating citizen science projects or those who aim to study or improve current practices in existing citizen science initiatives. The guidelines include challenges and applications of citizen science and recommendations for cities and citizen science initiatives on how to foster policy and behaviour change using citizen science. These guidelines result from a co-creation process led by Eurocities in close collaboration with the CitiMeasure Behaviour and Policy working group members from September 2021 to February 2023. The co-creation process included a joint effort to identify and share relevant resources and experiences and collaborative analysis, synthesis, and sense-making of the identified resources. 4