The paper first discusses certain legal matters germane to the issue of computers and evidence. An actual case, R v Pettigrew, is then described and the relevance of hearsay evidence is explained. The situation in certain common law countries is mentioned. Sources of errors in computers which could affect the accuracy of output, in terms of suitability for classification as ‘conclusive evidence’, are then considered as they might be used by a knowledgeable lawyer in court. Finally, there are some suggestions as to how DP professionals might prepare themselves for the role of expert witness.