In analysing the safety strategies of organisations successfully managing hazardous systems it is apparent that safety itself is a problematic, and even risky, concept. It is less the valuation of safety per se than the disvalue surrounding mis-specification, misidentification, and misunderstanding that drives reliability in these organisations. Two contrasting models of high reliability can be identified in precluded event and resilience focused organisations. Each model is adapted to different properties in the raw material, process variances, and knowledge base of the organisation. These two models bound the reliability approaches available to medicine. The implications of each for medical reliability strategy are explored, and the possible adaptation of features from each for medical organisations are assessed.