In the search and seizure context, the United States is much more heavily wedded to warrants and exclusion than European countries and in the interrogation setting requires more robust warnings than most nations in Europe. Comparative empiricism is an empirical assessment of the relative effectiveness of these types of differences between nations regulatory regimes. In the law enforcement context, this type of assessment might be the only realistic means of determining the combination of mechanisms that best protects against government over-reaching without unduly stymying good police-work. Domestic research that attempts to explore differing regulatory approaches either occurs in experimental settings that undermine generalizability or is constrained by national laws that prohibit or limit the ability to manipulate investigatory rules. In contrast, the significant country-by-country differences in approaches to police regulation, combined with the relatively consistent demands of police work across countries, provide a naturalistic setting for testing the effectiveness of a wide array of rules. In particular, comparative empirical work that uses the same metric for gauging effectiveness - this article proposes "hit rates" for searches and seizures and confession and clearance rates for interrogations - can provide a unique source of information to policymakers. After describing the law of search and seizure and interrogation in four European countries, Australia and the United States, this article examines the newest research studying the effects of these rules and lays out an empirical agenda.