The article focuses on the most successful period in the history of Soviet intelligence in the United States, namely the 1930s and 1940s. The reasons for this success are analysed, first and foremost being the worldwide enthusiasm for the ideas of communism and the achievements of the USSR in building a new socialist society, to which the propaganda of the Stalinist regime had contributed in no small measure. The author examines the activities of the Soviet secret services, which established an extensive covert network in the United States during those years. Members of the underground were collecting information, primarily in the field of the latest military technologies, including the secrets of the production of the atomic bomb. While the history of intelligence professionals has been sufficiently studied, the work of their American voluntary agents is less known. There were many communists and sympathisers among them; a significant proportion were Russian immigrants. The aim of the article is to explore their views, behavioural motives, and subsequent fate. The study draws on records from American and Russian archives opened to researchers in the 1990s: previously classified Soviet diplomatic correspondence, which, after being decrypted by the Venona project, was recognised as a communication channel between intelligence in the United States and the centre in Moscow; it was supplemented by the so-called “Vassiliev Notebooks”, containing documents from the archives of the Foreign Intelligence Service (formerly the First Directorate of the KGB) as well as records from the Comintern archive at the Russian Centre for the Preservation and Study of Documents of Contemporary History (RTsKhIDNI). New sources offer a more comprehensive picture of the scale and methods of Soviet intelligence work, the activities of American agents, and allow to answer a number of questions that have caused controversy among historians, including the guilt of the Rosenbergs in the theft of nuclear secrets and whether Alger Hiss, a high-ranking US State Department official, was a Soviet intelligence agent.