This report relates to the proof testing of safety instrumented systems (SIS). It has been produced as part of a research project funded by the Health and Safety Executive. The contract was placed with Eutech Engineering Solutions Limited. The objectives of the research were to define a set of guiding principles upon which proof tests of SIS should be developed, to provide independent advice to HSE Field Inspectors and to report on current practices, highlighting examples of good and poor practice where relevant. The research gathered data from industry via interviews, workshops and questionnaires and executed searches of data within the public domain. This report presents the findings of the research. The target audiences for the report are industrial end-users of SIS and HSE Field Inspectors. Industrial end users should benefit from the communication and illustration of current practices. The collation of experiences, views and examples give an indication of what is practicable and of the issues of concern to different sectors within the chemical industry. The data generated by the industrial research is presented ‘as found’ in an attempt to provide a genuine indication of the practices, experiences, and concerns within the industry. The data collected during the research was analyzed and the results used to generate and support a set of guiding principles for the proof testing of SIS. Each principle is presented in this report and is supported by the research data and a rationale based on recognized standards and guidance. The current high profile of IEC615081 amongst end-users was evident throughout the research. Its principles have been used to construct the rationales supporting the guiding principles. Detailed recommendations have been generated to provide practical guidance on the application of the guiding principles. The recommendations have been used to create a checklist for use by HSE inspectors. The checklist is presented in Appendix 2.