Witness proofing – or preparation – is an adjunct of the adversarial criminal trial process. It is also a common practice at the UN international criminal tribunals, where it has been repeatedly challenged, analysed, and endorsed. Recently, a trial chamber at the ICC prohibited the prosecutor from proofing witnesses, seemingly calling upon the institution, at an early stage, to break with the established practice of proofing at the UN international criminal tribunals. This article examines witness proofing in international criminal procedure with the aim of describing and weighing its relative merits, and arguing that proofing – as practised at the UN international criminal tribunals – appears to be a better modality for enhancing the efficiency, integrity, and legitimacy of the truth-seeking function of international criminal trials than does prohibiting the practice.