The ninth volume of Mykhailo Hrushevsky's History of Ukraine-Rus' is by far the longest in the ten-volume series. Written in the late 1920s, after Hrushevsky had returned to Ukraine from exile, the volume is based mainly on a wealth of documents gathered by Hrushevsky and his students in the Moscow archives. Many of these documents were little used or unknown to previous historians. The pivotal event in this part of the volume is the Pereiaslav Agreement of 1654, which brought Cossack Ukraine under a Muscovite protectorate. Needing military assistance to continue the struggle with the Polish-Lithuanian Commonwealth, against which the Cossack Host and much of the Ukrainian populace had rebelled in 1648, Hetman Bohdan Khmelnytsky was prepared to make an agreement that brought Muscovy into the conflict on terms favorable to the Cossacks. Hrushevsky analyzes the diplomatic and military developments that led up to the agreement, and in chapter 7 he presents the most detailed and thoughtful treatment in modern historiography of the Pereiaslav Council of January 1654 and the subsequent understandings with Moscow. In his discussion Hrushevsky deals not only with previous historiography and the documentary record, which is incomplete, but also with the negotiations, taking account of the conflicting motivations of the two sides. The subsequent chapters trace the difficult course of Cossack Ukraine's relations with Muscovy in 1654-55: the joint military campaign against the Commonwealth, which almost led to disaster because of poor coordination; the Cossack leadership's efforts to take control of the western Ukrainian and southern Belarusian lands; the ferocious battle of Dryzhypil; and the devastation of the Bratslav region by Polish and Tatar forces, against which Muscovy provided no effective protection. On the basis of the travel diary of Paul of Aleppo, a Syrian cleric, Hrushevsky gives an account of daily life in Ukraine at the time, with many details unavailable in other sources. Unparalleled in breadth of research, Hrushevsky's work brings to life a turbulent and politically decisive period in the life of the Ukrainian people. This volume was translated by Marta Daria Olynyk and edited by Serhii Plokhy (consulting editor) and Frank E. Sysyn (editor in chief) with the assistance of Myroslav Yurkevich. The volume includes an extensive historical introduction, a full bibliography of the sources used by Hrushevsky, 3 maps, and an index. The preparation of this volume for publication was funded by a generous donation from Mrs. Daria Mucak-Kowalsky (Etobicoke, Ontario) in memory of her husband, Mykhailo Kowalsky.