At first thought, iteration seems banal. It is about repeating the existing; nothing is changing. But this special issue shows that, in an era obsessed with the new, it is often the repetition of the old that creates social change. Iteration fosters persuasion. It affords opportunities for critical and creative engagement with meaning, values and knowledge. It invites collaboration, though its apparent simplicity often belies a tremendous amount of individual and collective labour involved in the practices of iteration. Through its repetition of the existing, however, iteration also can be a mechanism for reproducing the status quo. Its pervasiveness and banality naturalises power, and its mimetic qualities shrink spaces for critical distance and care. The editors of this special issue have brought together a delightful and fascinating diversity of articles focussed on iteration in cultural production in the digital age. We hop across geographies to examine lockdown diaries, artists' books, socialist memes, fake news, the design of social media platforms and artificial intelligence, activism, film, social media forum moderation, news website reader comments and more. Iterating through the collection as a whole, across its many disciplines, is a commitment to theorising through empirical evidence, to explaining with critique, and to providing pathways to praxis. These characteristics of this special issue, and the many concepts and arguments it puts forward, make this collection of work exceptionally rich material for seeing iteration and how it shapes the world we live in today, as well as the world we want it to be. In this preface, I take a media sociology approach to show how iteration can be usefully understood as collaborative communication for change. I see this understanding of iteration, whose ascendancy is related to the ascendancy of computer science, as baked into the form of communication technologies-and thus as shaping the kinds of iteration that are possible when we use these technologies. This understanding also prompts us to focus on the connection between iteration and social change. To explore how this works, I analytically slow down the practice of iteration to show that it is a communication practice of transmission. That transmission practice is itself constituted of cognate communication practices-the reception, evaluation and production of knowledge-in which visibility and persuasion are key. In the latter parts of the preface, I illustrate this through the example of witnessing as iteration, as the high-stakes nature of witnessing make it a canary in the coalmine, more generally, for mediated communication in the digital age. I show how breaking the witnessing practice down into its various parts allows us to see how power enters and inflects who and what are iterated, when-and who and what are not. Thinking critically with iteration and against unequal power relations, the praxis this preface suggests is one-much in line with the rest of this special issue-of explaining how iteration might move the grassroots towards their goals.