To ensure the correctness of publicity material ('truth in labelling') and to inform their licensing decisions, agencies certifying or regulating any clinical computer system will need information about the system's structure, performance and likely impact on users and the environment in which they work. This information must be reliable and complete, so it needs to be collected in a structured, rigorous evaluation programme. Clinical decision support systems are generally more complex and their effects less easy to predict than most other clinical software, so pose the greatest challenge to evaluators. They are therefore the focus of this paper.