The article is devoted to the study of bilateral relations between Azerbaijan and Iran in 1991-2019, as well as historiography on this topic. The author analyzes the priority areas of cooperation, such as trade and economic relations, tourism, cooperation on trilateral platforms, as well as the factors that influenced the dynamics of their development: Azerbaijani Turks living in Iran; the issue of the legal status of the Caspian Sea and the Nagorno-Karabakh conflict. The author also identifies six stages of bilateral relations: 1) building relations (1991- 1993); 2) trouble trusting in a relationship (1994-2000); 3) the most tense period (2001-2003); 4) enhanced cooperation (2004-2010); 5) sharp deterioration (2011-2013); 6) normalization and rapid development (2014- 2019). It is indicated that because both states had different foreign policy strategies and international positioning on regional issues, in 1991-2019, bilateral relations were characterized by inconsistency - cooperation, on the one hand, and confrontation, on the other. However, thanks to the efforts of the presidents of both states, bilateral interaction during this period was generally able to maintain its constructive nature. Special attention is paid to the cooperation in the regional triangles: Azerbaijan - Iran - Russia and Azerbaijan - Iran - Turkey, which had different goals and different expectations. If the trilateral cooperation between Baku, Tehran and Moscow is mainly aimed at the implementation of the North-South International Transport Corridor, then the cooperation between Baku, Tehran and Ankara is aimed at strengthening trust and confidence in interstate relations. In conclusion, the author stresses that thanks to the efforts of political elites, especially the Presidents of Azerbaijan and Iran, in general bilateral relations in 1991-2019 were able to maintain their constructive character. © 2021, RUDN UNiversity. All rights reserved.