The concept of the boundary plays a very important role within the philosophy of Georg Simmel, the philosophical anthropology of Helmuth Plessner, and the social systems theory of Niklas Luhmann. As a basic idea of their theories, the key concept of the boundary allows one to understand the specifics of human existence, the organizing principles of society and social differentiation, and relations between individuals in social interaction. The article tries to reconstruct the systematic content of the concept used by these thinkers, as well as its common roots, and both the similarities and differences between the three perspectives.