Many citizens enter the public sphere and politics to defend the interests and rights of others. Mobilisation to defend migrants, human rights, the climate, peace processes and the rights of those in vulnerable positions are just a few examples where collective actors and citizens are engaged for others, or more globally, for the common good. What are these movements of solidarity? Who are the citizens engaged in them and why? What is the impact of their actions? These are some of the questions that arouse our interest and which we wish to develop within the GREC. Questions linked to altruism and pro-social actions have always interested researchers, over a broad disciplinary range, from philosophy to biology, and including sociology, psychology, economics, and more recently neuroscience. For over a decade, research in this field has enjoyed a surge in interest. Such work allows us to question and redefine the individualist paradigm and helps explain how both individual behaviour and collective action develop in our societies.