In cyberspace, data flows transit massively and freely on a planetary scale. The generalization of encryption, made necessary by the need to protect these exchanges, has resulted in states and their intelligence services forgoing listening and interception missions. The latter have had to find ways to break or circumvent this protection. This book analyzes the evolution of the means of communication and interception, as well as their implementation since the advent of the telegraph in the 19th century. It presents this sensitive subject from a technical, historical and political perspective, and answers several questions: who are the actors of interception? Who has produced the recent technologies? How are the markets for interception means organized? Are the means of protecting communications infallible? Or what forms of power do interceptions confer?