Systematic and ad hoc cross-border exchanges of scientific evidence for law enforcement purposes are increasing. This chapter investigates what we ‘need to know’ when law enforcement agencies from one country seek to utilise scientific evidence produced in another jurisdiction. Criminal courts potentially face additional challenges, above and beyond those presented by all scientific evidence, when complex chains of custody and attenuated communications confound domestic attempts at ensuring the safety and reliability of scientific evidence. This chapter assesses the adequacy of national and international safeguards to ensure the reliability of scientific evidence generated extraterritorially. Subsisting regulatory frameworks and quality assurance mechanisms are described, with a view to assessing whether a ‘country by country’ approach secures ‘all we need to know’ about the provenance and probative value of scientific evidence. Alternatively, does international evidence exchange demand more comprehensive institutional solutions to guarantee evidential reliability and trustworthiness for the purposes of criminal adjudication?