I&C systems important to safety may be implemented using conventional hard-wired equip-ment, computer-based (CB) equipment or by using a combination of both types of equipment (see Note 1). This International Standard provides requirements and recommendations (see Note 2) for the overall I&C architecture which may contain either or both technologies. This standard highlights also the need for complete and precise requirements, derived from the plant safety goals, as a pre-requisite for generating the comprehensive requirements for the overall I&C architecture, and hence for the individual I&C systems important to safety. This standard introduces the concept of a safety life cycle for the overall I&C architecture, and a safety life cycle for the individual systems. By this, it highlights the relations between the safety objectives of the NPP and the requirements for the overall architecture of the I&C systems important to safety, and the relations between the overall I&C architecture and the requirements of the individual systems important to safety. The life cycles illustrated in, and followed by, this standard are not the only ones possible; other life cycles may be followed, provided that the objectives stated in this standard are satisfied.