Challenges for behaviour change: 15 Changemaking is one of the cross-cutting principles and purposes of citizen science initiatives. Change in actors’ behaviour, such as behaviour towards the environment, creating a new culture of collaboration and sharing responsibilities are among the aims of many citizen science initiatives. Changemaking is defined as changes in individuals, communities, cultures, and institutions, as well as in thinking, attitudes, behaviour, and values. Citizen science initiatives provide various opportunities for the co-production of knowledge, learning, gaining skills, change of attitude and behaviour, and communication with a wide range of audiences. Nevertheless, several factors may hinder such changes. Communities are heterogeneous, and each person has certain perceptions, priorities and needs that drive their behaviour. In addition, challenges that citizen science projects address are often complex and existing formal processes and informal norms define actors’ behaviour towards those challenges. Although several citizen science initiatives aim to or claim to have, changed actor behaviour, measuring such changes is not easy. Quite often, a baseline situation of actors’ behaviour before involvement in a citizen science initiative is missing. Moreover, a significant proportion of citizen science projects’ impacts, including changes in actors’ behaviour, happen after the lifetime of the projects and their funding period. It is therefore challenging to measure such changes.