In 1961, Leonard Kleinrock submitted to the MIT a PhD thesis entitled: "Information Flow in Large Communication Nets"1, an innovative idea for message exchanging procedures, based on the concept of post-office packet delivery procedures. It was the seed of ARPANET, a wide area data communication network, implemented in 1969, considered the origin of the Internet. At the end of the 1970''s, digital transmission and packet-switching allowed the building of ISDN (Integrated Services Data Networks). Voice and data were integrated in the same network, given birth to electronic offices combining computation and communication technologies. The electronic miniaturization and the popularization of micro-computers in the 1980''s, brought computer communication to home, allowing the integration and automation of many domestic tasks and access to some daily facilities from home. A new technological breakthrough came in 1989, when Tim Berners-Lee, a British scientist working at the European Organization for Nuclear Research (CERN), conceived the world wide web (www), easing the communication between machines around the world2. Nowadays, combining Kleinrock and Berners-Lee seminal ideas for network hardware and software, Internet became all pervasive in the daily life around the world, transforming the old telephone set into a small multipurpose computer. Consequently, human life radically changed. Our dependence on computer networks became undeniable and together with it, harmful programs or malwares, developed to damage machines or to steal information, represent permanent threat to individuals and society. In computer science a new work research line emerged: cyber-security which includes developing models, routines and software to protect machines and networks from malicious programs. This new discipline has attracted researchers to develop ideas for protecting people and corporations. Cyber-security is the object of this book, that presents hints about how the community is working to manage these threats: Mathematical models based on epidemiology studies, Control of malwares and virus propagation, Protection of essential service plants to assure reliability, the direct impact of virus and malwares over human activities and behavior, Government entities which are highly concerned with the necessary preventive actions. As cyber-security is a new and wide subject, the intention was to give a general idea of some points, leaving to the readers the task to go ahead.