M.A.D. again?

Anotace: The article introduces a new meaning of the acronym M.A.D. (Mutually Assured Destruction) in the field of security. From the original connection with nuclear issues, its use is moving to the area of cyberspace. The text introduces the basic division of domains important for human activities. It explains their importance in terms of undeniable usability and availability for mankind, at the same time, it draws attention to their vulnerability and the need to ensure their protection. It deals in more details with the new fifth domain – cyberspace. In this context, security threats are mentioned and some legal aspects are indicated.