This presentation provides an overview of the concept of a safety integrity level (SIL) and its importance in the structure and compliance requirements within IEC 61508 and associated standards. In particular, the presentation outlines the following: the failure categories within IEC 61508 (systematic failures and random hardware failures) and of their relationship to the concept of SIL; the reason why we need the concept of SIL; the relationship of SIL to achievement of the tolerable risk frequency; the concept of modes of operation and how this impacts on SIL determination. The presentation provides an understanding of the key concepts sufficient to support other presentations being given.