A landmark of Tartu semiotics for nearly half of a century now is the semiotics of culture. The semiotic study of cultures was at the centre of interest in Juri Lotman's group, and in the Tartu-Moscow semiotics school in general. In 1973, the collective manifesto Theses on the Semiotic Study of Cultures was written under the leadership of Juri Lotman together with his Moscow colleagues Vjacheslav Ivanov, Vladimir Toporov, Aleksandr Pjatigorskij, and Boris Uspenskij. This marked the real birth of the research field called the semiotics of culture. This volume contains the republication of Proposals for the programme of the 4th Summer School on secondary modelling systems, written by J. Lotman (1970), and the Theses on the semiotic study of cultures (1973), as well as a Postscriptum to Theses, written by J. Lotman and B. Uspenskij (1979). Both the Proposals and the Theses are presented in English, Russian and Estonian; the Postscriptum only in English. The background and the further role of these texts is described in the accompanying article by Peeter Torop and Silvi Salupere.