The article is devoted primarily to the analysis of Reinhold’s Letters on Kantian Philosophy, published in the “Der Teutsche Merkur” (German Mercury) 1786-1787. The purpose of this analysis is to reveal the peculiarities of Reinhold’s interpretation of Kant’s philosophy, as well as the possibility of the influence of these letters on Kant's practical philosophy and the further development of German idealism in general. To more clearly define Reinhold's own position, the article turns to Reinhold's early works, written before Letters, as well as to the 1790 and 1923 editions of the Letters. Particular respect is given to three topics that are closely related to each other: “Why did Reinhold’s Letters bring fame to Kant?”, “To what extent was Reinhold an independent philosopher?” і “What kind of infusion did his Letters make into the development of Kant’s philosophy and German idealism?” The article proves that before 1786, Reinhold had, in general terms, formed a program for the reform of philosophy, which was critical in its nature and had a clearly expressed practical focus. However, Reinhold's criticism was closer to the criticism of the eclectics and Leibniz than of Kant: it was aimed at overcoming the limitations of the original points of view and assumed a deep knowledge of the history of philosophy. The main reason for the popularity of Reinhold’s Letters lies in his idea of the “scientific revolution” in Germany, which should have become an alternative to social revolutions in other countries of Europe. Formed in “pre-revolutionary” articles, this idea received its justification in the second edition of the Letters. It was the nationalist idea of transforming the Germans into a “nation of scientists”, that became the reason for the rise of national consciousness in Germany and attracted “thinking youth” to it Reinhold's program for the reform of all philosophical sciences from a generally valid point of view gave impetus and largely determined the further development of German classical philosophy: not only Fichte, but also Schelling and Hegel relied primarily on his program. The first edition of his Letters largely determined the main themes of the Critique of Practical Reason, and the second - the Metaphysics of Morals. The second edition inspired Kant to develop a system of criticism and the basic ideas of Kant’s philosophy of law.