This report investigates how the immigration judicial review system is operating in practice. In 2013, most immigration judicial reviews were transferred from the Administrative Court to the Upper Tribunal (Immigration and Asylum Chamber). At the start of the study in 2017, there were some 12,000 immigration judicial reviews lodged each year with the Upper Tribunal (the caseload has subsequently declined). Judicial review is a critical mechanism for challenging immigration decisions. However, there is comparatively little detailed evidence on how it is used by claimants and how the system works in practice. Policymakers wanted a more detailed understanding of this area of litigation. We therefore undertook an empirical study to fill this important gap in the evidence. We did this by collecting data from a sample of case-files. We also interviewed Upper Tribunal Judges, representatives, claimants, and others. We were assisted by an Advisory Group comprised of representatives, officials, and an Upper Tribunal Judge.