In academic debates on the responsibility of international organizations and their member States the different identities of States play a crucial role. However, apart from the difficulty to clearly separate ‘State’ and ‘member State’ identities, it is even more complex to distinguish between the different roles ‘member States’ may have in the framework of international organizations. As a general introduction to this special forum, this essay aims to clarify the different identities and roles States may have in relation to international organizations, especially in the context of the responsibility of international organizations. As the subsequent contributions reveal, the law on the international responsibility of international organizations takes account of the possible responsibility of their members. By mapping the different identities States may have in different settings, this contribution argues that such differentiations may be crucial for the further development of adequate international rules on the responsibility of international organizations and their members.