Deviational analysis is a traditional way of exploring the safety of systems. The results of deviational analysis contribute to traditional safety cases and safety arguments. We extend deviational analysis to other aspects of dependability, notably security. We discuss how the evidence of deviational analysis can contribute to the validation of regulations, in the sense of their application of regulations to real systems. Keyword: deviational analysis, dependability, regulation validation 1 Background Regulations are intended to control the way that choice operates in critical systems. Validation must include consideration of how well their intent is met by real systems operating within the regulations. We describe the systematic analysis of security, illustrating it with results from a case study of the security of baggage handling in an international airport [11]. The case study was carried out in situ, with the co-operation of the relevant airport staff.