Publisher Summary This chapter explains the concepts of fault-finding and testing. There is no right or wrong way to fault-find in instrument loops and systems. There are however, safe and potentially unsafe ways that are of the greatest importance to consider. The chapter aims to discuss the approach to fault-finding in such a way that guidance can be given on what shall and shall not be done from an explosion-protection safety point of view. The approach is described here such that the areas where aspects of safety must be considered can be clearly identified. This is one of the possible ways of proceeding; it is not a standard recommendation but is distilled from good engineering practice and experience in very general terms. The testing of the different types of IS interface will be examined. The testing discussed concerns functional operation, as the integrity of the safety protection cannot be verified without access to internal components. If an IS-certified installed apparatus is tested on site, it will not reveal the condition or status of explosion protection. Testing is not a specific requirement of the codes of practice where the test is not able to check the protection integrity. Unnecessary testing is not substantially useful and can cause more harm than good.