Motivated primarily by the wish to bring clarity to the range of questions associated with the membership of the Estonian Communist Party (hereinafter ECP) in 1940–1941 (when, how many, and how people joined the ECP back then), material from six groups of relevant archival sources was used within the framework of this research paper. By putting together fragments of information from the transcripts of local ECP committee sessions, from ECP membership cards, from the transcripts of the ECP Central Committee Bureau, from the evaluation statements of Party members issued by local committees, from the books where transcripts are recorded of the issuing of CP(B)SU (Communist Party of the Soviet Union) membership cards and candidate cards, and from the personal files of Party members, it became possible to put together a time series from the summer of 1940 to the summer of 1941 of people joining the Communist Party in Estonia (the acceptance of new, local people). The results thus obtained do not refute the results of research obtained by Soviet historians of Communist Party history, but they aim to make these results more precise (the quality of Communist Party statistics for 1940-1941 leave something to be desired). The materials gathered from the above-mentioned sources similarly help to more precisely describe the large-scale processes that were carried out within the Party in 1940-1941, such as the evaluation of Party members and the distribution of Party documents, information on which has similarly been deficient.