Abtract: Although prison inspection in the United Kingdom has a long history, inspection of immigration detention was properly established only in 2004. Her Majesty’s Inspectorate of Prisons (HMIP), a government-appointed independent human rights-based monitoring institution, holds this responsibility. In this GDP Working Paper, a lead HMIP inspector discusses the nature and impact of the Inspectorate’s work, examining both the theory and practice of inspection. The paper places the discussion in the broader context of prison reform and debates on migration and border controls. The author argues that in liberal-democratic societies there are two broad approaches to promoting human rights reforms and challenging abuses: working from the inside to achieve progress with the risk that principles may be compromised and good intentions confounded; or promoting change from the outside, which is more uncompromising but less influential, at least in the short-term. This is a dilemma that confronts human rights based inspection of immigration detention in the UK. The main focus of HMIP is on improving the treatment of detainees and conditions in detention, not challenging the system of detention, even if immigration detention policy arguably lacks legitimacy in a way that criminal imprisonment does not. The author explores the “effectiveness” of detention inspection and whether inspection can be said to have promoted meaningful change.