The paper describes the role of Independent Safety Auditor (ISA) as carried out at the present in the defence and other sectors in the UK. It outlines the way the ISA role has developed over the past 15-20 years with the changing regulatory environment. The extent to which the role comprises audit, assessment or advice is a source of confusion, and the paper clarifies this by means of some definitions, and by elaborating the tasks involved in scrutinising the safety argument for the system. The customers and interfaces for the safety audit are described, and pragmatic means for assessing the competence of ISAs are presented.