Digital information and data processing, storage and transmission are already at the core of most modern enterprises and most individuals have significant digital footprints. Computer-based information networks operating in cyber-space (interconnected on the Internet) are at the core of modern businesses many of which operate across countries and continents. Government and human development enterprises (health, education, etc.) depend critically on Internet-based operations. The traditional systems of in-house applications and data storage are rapidly being replaced by shared or independent Cloud services. However, these highly beneficial developments in information technology also come with a variety of cyber-threats. The risks may originate from personal cyber-habits, employees, clients and contractors, or external cyber-criminals; they may result from deliberate acts or human errors. Irrespective of the source or cause, the consequences can be devastating, ranging valuable or sensitive data loss, or disruption of operations of sensitive infrastructure. Cyber-crime is increasingly weaponized to extract ransom payment or cripple sensitive infrastructure of enemy nation states. Cyber-security has emerged as a major technology discipline and, with the exponential rate of personal and corporate migration to cyber-space, incidents of cyber-crime are projected to grow at a similar rate. This introductory book presents a comprehensive overview of the digital cyber-space, evaluation of the extent of cyber-threats, the critical information technology practices and infrastructure that facilitate cyber-attacks, the main criminal actors and their strategies, and current status and trends in cyber-defense strategies for protecting the digital world.