The book Online Virality, edited by Valérie Schafer and Fred Pailler (C2DH, University of Luxembourg), aims to provide a comprehensive examination of online virality. It explores the many ways we can think about this modern phenomenon and analyse the circulation, reception, and evolution of viral born-digital content. Virality and content sharing always intertwine material, infrastructural, visual and discursive elements. This involves various platforms, stakeholders, intermediaries, social groups and communities that are constantly (re)defining themselves. Regulation, curation and content moderation politics, as well as affects and emotions (fears, humour, empathy, hatred...), are also at the core of online virality. The publication offers an interdisciplinary overview on online virality by including different types of scientific inputs, such as precise case studies, various methodological approaches (including close and distant reading, visual studies, discourse analysis, etc.), as well as historical and socio-technical analyses. The book is organised around three main topics: Expressions and Genres; Mobilisations and Engagements; Circulation and Infrastructures. The first part explores the semiotics of virality, the diverse and creative forms of expression, specific genres, the relation to other media, and the affective side of virality, such as using humour or provocation. The second part focuses on the political dimension of memes and viral content and their use in the context of controversy or political and ideological opposition. Finally, the third part delves into the often understudied but essential side of virality, by examining the role of platforms and their curation, in short, the infrastructural dimension of virality. These three parts allow us to question such fundamental notions linked to virality as, among others, circulation, reception, economy of attention, instrumentalisation and affect. This volume brings together authors from various disciplines, including semiotics, history, information and communication sciences, computer science, digital humanities, media studies. In addition, the contributors approach the question via case studies that allow for a perspective that is not exclusively US and European-centred. Some chapters explore virality in Brazil, Chile, while the book also examines a wide variety of platforms (YouTube, Twitter, Instagram, TikTok, video game platforms, etc.).