This International Standard provides requirements for the software of computer-based I&C systems of nuclear power plants performing functions of safety category A as defined by IEC 61226. According to the definition in IEC 61513, I&C systems of safety class 1 are basically intended to support category A functions, but may also support functions of lower categories. However the system requirements are always determined by the functions of the highest category implemented. For software of I&C system performing only category B and C functions in NPP as defined by IEC 61226, requirements and guidance of IEC 62138 are applicable. This standard provides requirements for the purpose of achieving highly reliable software. It addresses each stage of software generation and documentation, including requirements specification, design, implementation, verification, validation and operation. The principles applied in developing these requirements include: – best available practices; – top-down design methods; – modularity; – verification of each phase; – clear documentation; – auditable documents; – validation testing. Additional guidance and information on how to comply with the requirements of the main part of this standard is given in Annexes A to I.