As is well known, the main difference between the Imāmiyya and Zaydiyya branches in Shīʿī Islam is to do with the fact that the Zaydiyya—named so after their first leader Zayd b.ʿAlī b. al-Ḥusayn (d. 122/740)—did not unconditionally condemn the first three caliphs before ʿAlī b. Abī Ṭālib, while to the Imāmiyya branch, all Sunnīs were infidels. But even though the Zaydīs did not consider Sunnīs generally as infidels, they regarded rebellion against Sunnī rule—unlawful to them—as a religious duty for all. The Imāmīs on the other hand, while radical in doctrine, did not have a militant attitude comparable to that of the Zaydīs. Geographically, the Zaydīs divided into a Yemeni and an Iranian branch, concentrated along the shores of the Caspian sea. The present work contains the biographies of 15 Zaydī imams, some from the Caspian, the author—Abū Ṭālib Hārūnī (d. 424/1033)—being a Zaydī scholar from that region.