This manual provides an overview of security in telecommunications and information technologies, describes practical issues, and indicates how different aspects of security in today's applications are addressed by ITU-T. The manual has a tutorial character: it collects security related material from ITU-T Recommendations into one place and explains respective relationships. The manual covers additional aspects of security, in particular those that relate to availability – for which ITU-T has a great deal to offer–and to environmental damage in which area ITU-T is also active. It also includes results achieved on security-related standardization since the second edition. Further, aspects covered are based on existing work, not on work in progress, which will be addressed in future editions of this manual.