Computer validation should address the scientific correctness of the application software, the business objectives of the organisation, and the concerns of regulatory agencies. It is a balance between a practical and cost effective system that must develop the confidence that the system is under control. This paper gives an overview of computer validation from the following perspectives. What is computer validation? Why can a computer system not be completely validated? Why and how should computer systems be validated? The principles outlined in this paper should be adapted to a specific system on a case by case basis depending on its scope and complexity.