Unlike the activities of flight crews, the work of Soviet military aviation engineers in Spain during the Civil War (1936–1939) has, until now, received little sustained scholarly attention; this article offers what is presented as the first systematic study of the subject in both Russian and non-Russian historiography. On the basis of an extensive body of archival documentation, specialized technical publications, and memoir literature, the author demonstrates that a number of Soviet specialists served not merely as advisers but, in fact, headed the aviation engineering service of the Republican Air Force as a whole, as well as its engineering units on particular fronts. This constitutes a rare case in military history and attests to the high professional competence of the Soviet aviation engineers dispatched to Spain. Soviet specialists directed the Republican aviation engineering service for nearly two years. This situation arose, first, because from October 1936 until the end of the war Soviet aircraft formed the backbone of the Republic’s air fleet; and, second, because among the small number of Spanish Air Force career officers who remained loyal to the lawful government there were no experts of sufficient qualification to organize independently the technically sound operation and repair of aircraft supplied from the USSR. The author identifies four Soviet advisers who held senior responsibilities for the air force’s aviation engineering service, while two others led the service on the Eastern (Aragon) front. The article describes the objective difficulties Soviet aviation engineers faced in establishing and administering such a service in a foreign country and delineates both the variety and the scale of their duties in Spain. These engineers were required to train Spanish engineering and technical personnel, to organize fixed and mobile aircraft -repair workshops, and to restore to operational readiness aircraft that certain countries sold to the Republican government without armaments or specialized equipment. The study thus brings into view their contribution to the creation and maintenance of a highly capable air force for Republican Spain. Particular attention is given to contemporary evaluations of the Soviet senior advisers’ performance – both by the head of the Spanish Air Force and by the Soviet military-political leadership.