Readers are invited to read a book by the prominent German public figure and publicist Karl Kautsky (1854-1938), dedicated to the study of the emergence and development of communist movements in Europe in the Middle Ages. The origins of communist ideas in antiquity are traced, in particular in the works of Plato. The economic, social and ideological foundations of communism in the Middle Ages and in the era of the Reformation are considered, the general nature of medieval communism is revealed. Much attention is paid to the study of the so-called communist sects that existed in different countries of Europe.