Empirical work in international law is rapidly increasing in quantity and sophistication. This trend reflects the expansion in number and importance of international organizations and courts, as well as developments in legal scholarship and the social sciences. This bibliographical essay forms the basis for a forthcoming chapter in the Oxford Handbook on Empirical Legal Studies (forthcoming 2010). It surveys empirical work on international tribunals, treaties and many substantive areas of law. Some of these areas, such as trade, investment and human rights, are the subject of burgeoning empirical literatures. Others, particularly private international law, have received less attention to date, but good work is beginning to emerge, as is the case with criminal law and investment law. We see the field continuing to expand and diversify in years to come, as many outstanding questions beg for analysis.