This chapter describes the role of international law in cyberspace and in addressing the legality of cyber-related actions taken by countries, companies, associations, and citizens on the world stage. Cyberspace has been described as a new ‘domain’ that is radically different from the land and sea upon which international law developed, but such law either covers or is evolving to cover issues that have been under development for hundreds of years. Such coverage includes the international law principles of sovereignty, due diligence, jurisdiction, and state responsibility. International law also covers numerous specialized regimes: human rights, diplomatic and consular law, law of the sea, air law, space law, and international telecommunications law. Another developing area of international law and cyberspace concerns international peace and security with respect to cyber activities, but those issues lead right up to the threshold of conflict. The once theoretical and purely academic literature of cybercrime, cyber espionage, and, especially, cyberwar must now be applied to real-world cases, and law and policy for the future of cybersecurity must be formulated with an understanding of these broadly accepted international norms.