This report provides practical guidance on the methods available for verification of the software and validation of computer based systems in nuclear power plants, and on how and when these methods can be effectively applied. It will be of particular interest to all those involved in the development, implementation, maintenance and use of software and computer based instrumentation and control systems in nuclear power plants. Contents: 1. Introduction; 2. Safety classification and types of software; 3. Software related activities and documents; 4. Verification by phase; 5. Validation; 6. Licensing; 7. Conclusions; Appendix I: Techniques for verification and validation; Appendix II: Single failure criterion; Appendix III: Experience.