Illegal, unreported and unregulated (IUU) fishing indicators apply readily available data and straightforward calculations to measure something that provides insight into – but does not actually estimate – the level of IUU fishing. In doing so, indicators aim to evaluate and monitor performance in combating IUU fishing, including highlighting where improvements in monitoring, control and surveillance (MCS) systems are needed. Since they are easy to update and track over time, indicators offer an alternative to often resource-intensive quantitative IUU fishing estimation methods, and can be adapted across a broad range of fisheries. This document reviews various types of indicators and develops a universal framework based on detection, coverage and investment indicators for IUU fishing. It then presents four hypothetical case studies, illustrating how these indicators can be applied to different fisheries with different IUU fishing risks. Guidance is provided for customizing indicators to individual fisheries and for interpreting various combinations of indicators moving in different directions. However, IUU fishing problems can have multiple drivers, take various forms and produce mixed signals, making it difficult to provide interpretive guidance that is appropriate for all situations. For this reason, a variety of recommendations are presented for engaging stakeholders and drawing inferences from similar fisheries.