A case study of the development of the Japanese Communist movement in relation to Japanese society, with particular emphasis on the period since 1945 and on the reasons why the Japanese Communist Party is not today a significant political force. The study examines the ideological and programmatic evolution of the party, its organizational and socio-economic structure; its functional operations as related to other political forces in Japan, particularly the forces of the left; and its changing attitudes toward the United States, the Soviet Union, and Communist China. It is concluded that the critical issue is whether the party can achieve a united front with the more powerful forces of the Japanese Socialist left and thus exert pressures to shape futurepolitical issues and tactics. The study is based in part on the author's personal interviews with a number of Japanise Socialist and Communist leaders. Comprative election statistics are tabulated.