Safety Instrumented Functions (SIF) have become a critical part of our hazardous systems. If we rely on them in some way to manage risks or wish to take credit for them when deciding if risks are acceptable they need to be tested. This task is not as simple as it may seem at first. Harvey Dearden and Nick Wise co-authored a paper with me on this subject for Hazards 32 in 2022. A shortened version was subsequently published in the Loss Prevention Bulletin in 2023. An overarching task and human error analysis formed the basis for some of this work and can downloaded here.