After finishing my mandate as chairman of the United Nations International Law Commission’s Open-Ended Working Group on the topic obligation to extradite or prosecute (aut dedere aut judicare) in 2014, I was interested in taking up the topic protection of personal data in transborder flow of information, which had been put on the International Law Commission’s long-term programme of work since 2006. However, I have eventually concluded that this latter topic is too narrow in its scope to meet the pressing needs of the international community. Cyberspace has become one of the domains for everyday human interaction in almost all corners of the earth. Clandestine cyber surveillance, cyber crimes, and cyber espionage are now common phenomenons. While cyber warfare resulting in large-scale catastrophes and cyber terrorism may seem remote possibilities, in the age of fast-developing cyber innovations there is no reason for complacency and international law must be ready to meet any present and future challenge. At international meetings both inside and outside of the United Nations system, States have expressed their positions on cyber activities and sought international legal bases to support their respective positions. Most international lawyers and governments also agree that the relevant rules of existing international law regulate cyberspace. Opinions diverge, though, on which rules are to be applied and how. This book will endeavour to provide practical and objective answers to the cyber-related international legal issues considered to be of importance by States, international organizations, individuals, and corporations, as well as other actors. Being one of the 19 members of the International Group of Experts of the NATO Cooperative Cyber Defence Centre of Excellence entrusted with writing The Tallinn Manual 2.0 on the International Law Applicable to Cyber Operations, or Tallinn Manual 2.0, has immensely enriched my knowledge about the international law governing cyberspace. Although this book of mine is intended for different readers and focuses on broader issues than the Tallinn Manual 2.0, I am beholden to all my colleagues in the Group since our brainstorming sessions have clarified so many aspects of the law.