The ＇Seveso II Directive＇ (96/82/EC) requires certain establishments holding hazardous substances to implement a Safety Management System (SMS). The first part of the paper discusses the origin of this requirement, the guidance drawn up to help companies fulfil it, and the results of preliminary testing of this guidance. One important element in implementing and inspecting SMS is the identification of SMS weaknesses, the safety culture that effects SMS and finally the evaluation of safety performance. The second part of this paper examines some principles underlying industrial practice in evaluating SMS. Audit-related activities, involving proactive performance indicators for monitoring compliance with standards, are used nowadays to identify weaknesses of the SMS. Preventive actions to control risks and policy updating within the company through strategic plans are found to be sensitive to the number and type of SMS elements taken into account as indicators. The value of quantifying performance using simple rating systems in audits may be limited to the single installation. The guidelines and evaluation methods could thus produce useful results only when safety aspects and system elements are identified and evaluated separately for each installation; the value of quantification is limited to the individual elements after considering their functional relation with the rest of the elements in the SMS environment and the local safety culture into which they are developed.