In this chapter, the authors set out some ideas about how comparative empirical research into the variations in police–prosecutor relationships in European jurisdictions might be structured, and how data about them might be analysed. They focus particularly on the allocations of these responsibilities between police and legally qualified prosecutors as prescribed in law as well as occur in practice, the consequent relationships between police and prosecutors in fulfilling these responsibilities, and to what extent there is some judicial oversight of the exercise of them. The authors also focus on the first and second, pre-trial, investigative and prosecutorial phases. The investigative function is essentially forensic and relies mainly on technical expertise and coercive powers, but also requires some legal knowledge/understanding. The prosecutorial function involves more normative "public interest" considerations requires a high level of legal, and also requires a more balanced review of the available evidence, including evidence that might exculpate the accused.