Court interpreters have seldom been featured in studies on the criminal courts. Until recently, cases requiring court interpreters were rare and marginal. The peculiarity and historical rarity of these cases may explain the lack of academic consideration of the work of court interpreters in the criminal justice literature. Rapid demographic changes brought about by mass migration, however, are changing the make-up of criminal justice proceedings, rendering court interpreters key participants and inexorable aides for the everyday running of the criminal justice system. This article examines the increased reliance on interpreters and the nature of their involvement in criminal justice proceedings. It will explore the relationship between interpreters and defendants, on the one hand, and between interpreters, counsels, and judges, on the other. Drawing on empirical data stemming from a research project on foreign national defendants conducted in Birmingham’s criminal courts, we explore issues of trust and reliability underpinning the intervention of court interpreters and the implications of these interventions for the defendant’s case. The use of interpreters aims first and foremost to ensure the defendant’s right to defense. Yet, as we show, their intervention is often propelled or hindered by instrumental, procedural, or logistical reasons, intimately linked to the rapid transformation of the demography of defendants and the privatization of services related to the criminal justice system.