In this paper, the author explores the historical significance and complexity of legal customs in the formation of legal systems. The paper analyzes the evolution of a custom from the primary sources of law in ancient societies to its modern status, where their role, although important, is subject to certain limitations, is analyzed.The paper highlights the understanding and application of the term «custom» in the context of modern civil legislation of the Russian Federation with an emphasis on the changes that occurred after the legislative reform of 2012. The study examines various fields of application of customs. Special attention is paid to the analysis of the role and place of international trade custom in civil law relations in the Russian Federation. Based on the opinions of experts and considering in detail the judicial practice of the International Commercial Arbitration Court at the Chamber of Commerce and Industry of the Russian Federation, the author concludes that international trade custom is a source of law, and it also falls under the concept of «custom» within the framework of Article 5 of the Civil Code of the Russian Federation. The study emphasizes the oral nature of a custom as a source of law, while not neglecting the importance of informal written forms such as Incoterms and UNIDROIT, as well as standard contracts Special attention is paid to the debatable topic of the relationship of customs with imperative and dispositive norms of legislation, as well as their place in the structure of the Civil Code of the Russian Federation. The author highlights the problem of insufficient legislative regulation of customs and calls for their further research and systematization as a critically important element of the legal system.