This article presents a critical review of the monograph «The Thin Blue Line: Gendarmes and Society at the Twilight of the Empire» (2023) by the historian of Russian political surveillance Anastasia M. Lavryonova. The book is evaluated in the context of recent achievements in Russian historiography on the prerevolutionary security services and the archival discoveries of recent years. The reviewer concludes that, despite several gaps in its treatment of the history of the Separate Corps of Gendarmes, the monograph is written to a high scholarly standard and incorporates an extensive body of quotations from previously unpublished archival documents. Lavryonova addresses a wide range of contested issues concerning the history of the law enforcement institutions of late imperial Russia, including the functions of the Gendarmerie Police Administrations of the Railways, the relationships between provincial governors and central and local police authorities, and the diverse spectrum of public responses to the activities of the gendarmerie. The monograph also briefly considers the extent to which the work of provincial gendarmerie administrations corresponded to the methods employed by the security sections. Another important issue examined is the attitude of provincial gendarmerie commanders toward the use of confidential informants. Among the book’s principal strengths are its statistical data on the size of the gendarmerie and the number of students involved in public disturbances, as well as its incorporation of material from pre-revolutionary periodicals and contemporary memoirs. At the same time, the reviewer expresses reservations about the insufficiently critical treatment of certain memoir sources cited in the monograph. Overall, Lavryonova’s study represents a significant contribution to the continuing scholarly investigation of the investigative and security institutions of the Russian Empire.