This book provides a comprehensive examination of how digital platforms shape deliberative processes and democratic engagement, combining theoretical insights with empirical evidence. It draws on typologies, conceptual models, case studies, comparative analyses, and ethical reflections to assess how digital tools can support—or undermine—the legitimacy of deliberative procedures and wider democratic participation. This book focuses on initiatives by public institutions to involve citizens in areas such as climate governance, youth engagement, public service media, party politics, and AI-mediated dialogue. Using the input-throughput-output legitimacy framework, it systematically explores how digital platforms influence who participates, how they engage, and how these processes connect to the broader public and contribute to policy outcomes. In doing so, this book equips readers to critically navigate the evolving landscape of participatory and deliberative practices amid rapid digital transformation. This book is essential reading for scholars, students, and practitioners interested in the use of digital technologies for participatory and deliberative purposes, and more broadly in political communication, democratic innovation, media studies, party politics, and the social sciences