A growing number of studies in international relations use experiments. Roth (1995) discusses at least three purposes for experiments: (i) testing theoretically derived models, or ''speaking to theorists''; (ii) data generation, which he called ''searching for facts,''; and (iii) ''searching for meaning,'' referring to policy applications, which he described as ''whispering in the ears of princes.'' Experiments in international relations have typically focused on the ''searching for facts'' type of experimental function and on theory (and hypotheses) testing.The main substantive contributions of experiments in international relations have been: (i) Experimental research has tried to shed light on both microlevel and macro-level behavior in international relations, often using different manipulations, samples, and populations, to explore these phenomena; (ii) Experiments, as heuristic devices, have also contributed to the emergence of new theories in international relations and foreign policy analysis, especially poliheuristic models; (iii) Experimental research has also provided cross-validation for findings derived primarily from other methodologies, such as statistical analysis, case studies, formal analysis, simulations, and computational modeling.In this introduction, we distinguish between: 1. Paper-and-pencil experiments, 2. Survey-based experiments, 3. Computerized process-tracing experiments, 4. Field experiments and natural experiments, and 5. Cognitive neuroscience experiments. 1 Authors' note : We would like to thank Eran Halperin of the Lauder School of Government, Diplomacy and Strategy at IDC-Herzliya, Israel for his helpful comments and suggestions.