The word internationalism originally referred to relationships between nations and states, but came quite early to mean relationships of solidarity between people and peoples across or despite national boundaries, inter-state conflicts and economic competition. Over the past few centuries it has been a constant feature of social movement practice, from the 1649 Leveller mutiny against joining Cromwell’s campaign in Ireland to the 1984 – 87 Dublin shopworkers’ strike against handling South African produce - or contemporary international solidarity with struggles in Mexico, Kobane or Ferguson. International solidarity has been hugely important in changing the terms of politics. External supporters often provide crucial sources of legitimacy, publicity, funding or knowledge – but they also tell local activists that they are not alone, that what they do resonates on a world stage and that official attempts to dismiss their issues do not convince everyone. Conversely, supporting struggles abroad can be a tool for educating movement participants, thinking outside the particular state’s political discourses and arrangements, and seeing other, more emancipatory possibilities. It is not only that together we are stronger; as movements make links outside local power arrangements they come to define a different kind of power, spoken more on their own terms than on those of the national state, the local wealthy, the dominant culture, and so on. What is hegemonic locally is often shown to be a provincial peculiarity on a wider scale – and hence contingent, vulnerable to popular pressure. For all of these reasons, social movements regularly think and act in international terms. At the same time, the practice of internationalism is anything other than straightforward. It exposes participants to particular pressures, from accusations of being foreign agents to isolation from the wider community; it can involve taking sides in often less than transparent internecine struggles of movements elsewhere; when successful, its effects are not always as expected; and the inequalities which often exist between participants can lead to bruising experiences. Over the years, Interface has published several discussions of transnational solidarity as well as many pieces which arise out of internationalist activism and research; as a project, of course, it is programmatically international, geared towards “learning from each other’s struggles” in different regions of the world – and organised on the basis of autonomous regional groups of editors. This special issue, we hope, takes the theme further with a thought-provoking selection of pieces.