In the Jewish legal system, courts composed of a panel of esteemed rabbis make legal determinations called responsa. These panels are called batei din (sing., beit din). Compilations of responsa are a type of post-Talmudic literature and a primary source of law. There are three primary historical periods of interest during which authoritative responsa were written: the Geonic Period (800 CE to around 1050 CE), the period of the Rishonim (1050 CE to 1500 CE), and the period of the Acharonim (the 16th century to the present day).
Researching responsa is challenging, as there is no single index or digest incorporating all known responsa (of which there are hundreds of thousands). However, the following online resources will assist the researcher. For more resources, see the Books, Journals, and Websites tabs respectively.