The commonly practiced approach to the safety assessment of complex technology, including those in the High Speed Railways1 lacks a supporting theoretical foundation as a guiding and supporting backbone. In practice, this results in a confused, poorly conceived and often inadequate application of a mixed bag of methodologies, rules and standards that, due to the effort-intensive nature give a semblance of adequacy and completeness. In this uncharted and poorly structured landscape, demonstration of compliance with a given rule or standard is broadly regarded as adequate input to the safety assessment, potentially missing other analysis, effort and evidence. The key aim of the research outcomes presented in this paper is to give an overview of the principal requirements and qualities for robust, credible and systematic assessment to be supported by a host of relevant processes, rules, tools, codes of practice and standards.