This Handbook offers a state-of-the-art overview and comprehensive analysis of the emerging field of cyber diplomacy. During the last twenty years a complex cyber ecosystem has been emerging that is increasingly challenged at various levels, in different domains and by a variety of actors. Core issues range from cyber (dis)- information warfare and frequent cyberattacks on critical infrastructure to sophisticated cybercriminals penetrating systems for income or simply securing rights online. Such challenges are not only of a strictly technical nature, but have also important social, economic, legal and geopolitical implications. This broad policy agenda can neither simply be addressed by states alone nor by traditional diplomats that practice or engage in negotiations over securing cyberspace, whether this is in relation to regulations, norms, rules or indeed technologies that can provide security and preserve fundamental rights and freedoms on the Internet. Developments in practice and in theory require more complex conceptualisation and understanding of cyber diplomacy: of what it is, of who practices it where and how? The Handbook seeks to contribute to the wider question on how cyber diplomacy might have affected and changed the tools and approaches of diplomacy itself and might influence the study of diplomacy in the future.