The chapter introduces and explains some terms and concepts that are important for the understanding of the international legal questions with regard to transnational cybersecurity, such as hacking and software vulnerabilities, exploits, phishing, ransomware, or botnets. Furthermore, some basic categories like espionage, cybercrime, or cyberterrorism are laid out in order to distinguish the most prevalent forms of malicious behaviour in cyberspace and to conceptually prepare the subsequent focus on adversarial state conduct in cyberspace.