Today, the dialogue is regarded as a basis for cultural being, while the dialogue of cultures has become a key notion in modern philosophical thinking. The concept of dialogue has been transformed over the past century, acquiring new meanings and changing its internal content from understanding it as an ordinary exchange of information to a complex creative interaction and mutual influence of different cultural and value consciousnesses. Not only different personalities, but entire ethnoses, cultures, and civilizations may become subjects of the dialogue, thus increasing the dialogue functionality up to the means of developing inter-cultural, inter-ethnic and inter-civilizational relations and accentuating commonality of the global historic process and cultural heritage of mankind. Appearing as a form of interpersonal relations in the ontology of M. Buber, who was one of the first to focus on the transition of relations from "subject-object" to "subject-subject", the concept of "dialogue" has become an important philosophical concept throughout the mid-XX century. Brand new turn of development of the theory of dialogue, and the entire human culture in General, was due to the concept of Semiosphere Yu.М. Lotman. The article deals with genesis of the philosophical concept of the dialogue between cultures in the 20th century. The focus is on its emergence - in the early 20th century - in M. Buber's theological concept and at the highest point of its development in Yu. M. Lotman’s semiotic philosophy.