The diary of Sergey Sergeyevich Dmitriev, a famous specialist in the history of Russian culture and social thought of the 19th century, is currently being prepared for publication. The purpose of this article is to show the relevance and use of this source of personal nature for potential readers (both historians and those who are simply interested in Russian history). The value of the diary can not only be attributed to the fact that its author was a professor who left a noticeable mark on the history of the Faculty of History at the Moscow State University, but also to the fact that it covers a half-century period, stretching from 1941 to 1991, and further into the 1920s-1930s, if we take into account the so-called memoir inclusions. During this time, the social life of the country and the conditions for the development of historical science changed more than once. The period of the 1920s - early 1930s, when the “Pokrovsky school” domineered, significantly differed from the situation prevailing after 1934, when the study of civic history was reintroduced in addition to the history of class conflict; whereas the conditions of the 1930s - early 1950s varied from those in which historical science developed after 1956, and then during the period of perestroika. All these changes can be tracked both on the basis of the assessments and judgments expressed by Dmitriev himself, and drawing on the example of his numerous contemporaries’ experiences mentioned in the diary. They were people of different ages, professions, social (class) origin, widely known or almost unknown to anyone; those who lived and worked with the author for a long time, and those who only encountered him for a brief moment, during the period of evacuation, or while he travelled around the country, stayed in sanatoriums and hospitals, or attended classes of various Moscow universities. Of particular interest is the correspondence between members of the Dmitriev family from 1941-1942, as it vividly and thoroughly characterizes life on the Soviet home front in the initial period of the Great Patriotic War.