In this lecture, data security will be explained from a conceptual, technical, operational and policy perspective. The concept section provides the value of data and the concepts of security goals such as confidentiality, integrity, and availability. The technical section focus on cryptographic theory and key management and deepen the understanding of the basic technical terms and their understanding. The operation section provides actual examples in public systems, financial systems and control systems, which are examples of social implementation of data security. The policy section provides an overview of the latest status regarding data security in Japan, the United States, and Europe.