Online activism showcases how available digital tools allow social movements to emerge, scale up, and extend globally by fundamentally enabling new forms of power. This special issue brings together eight research articles that engage with the collaborative efforts embedded in various types of activism by studying features such as the socio-technical systems involved; how the activism is organized; relations between traditional and social media; and the complex network of systems, information, people, values, theories, histories, ideologies, and aesthetics that constitutes such activisms. The articles show the spaces in which this activism materializes, and particularly their situation in several intersecting dimensions including motivation, culture, language, and infrastructure. Together, these articles reflect the methodological breadth required to materialize online activism and the need to develop a more nuanced conceptualization of the media ecologies involved. By mapping out how activism is enabled and constrained by human-computer interfaces, this special issue contributes to open up the black box of online activism.