In this paper, we provide a concise review of 20 methodological approaches to measuring irregular migration for a non-technical audience. First, we revisit established approaches covered in previous reviews (such as CLANDESTINO). Second, we review innovative approaches which apply new methodologies or make use of novel data. We include any empirically tested attempts to measure irregular migration stocks or flows. Each approach is discussed in the form of a one-page bulletin that describes the main idea behind the approach, the required data sources, the definition or coverage of irregular migrant groups, the reliability and scalability of the approach, the main assumptions of the approach as well as potential ethical issues for researchers. We find an increasing diversity and complexity of approaches that made important advances to overcome limitations or drawbacks of traditional approaches. Despite the progress made in estimating irregular migration in the last decade, approaches remain highly fragmented depending on the type of data source used for the estimation. Some approaches can only be applied in specific countries due to data limitations. Any evaluation of the relative performance of various approaches is still missing due to the absence of comparative assessments testing various approaches for the same context. Simulation studies could offer a way forward. One trend in the last decade has been the emergence and use of digital data which offers promising avenues for future research.