Human communication is based on mutual interaction between participants. Much of this communication is linguistic in nature. Language is structured by grammar and grammar is inherently metonymic (Langacker 2009). Thus, language and interaction must be metonymic. This article explores the metonymic basis of human interaction in both its linguistic and non-linguistic aspects. First, it makes a distinction between linguistic and cultural metonymy. Both have a conceptual basis. The former, extensively studied from the view of cognitive linguistics, has a linguistic source. The latter, found in fields as diverse as art, theater, and film, does not necessarily have a linguistic source. The broader concept of cultural metonymy seems to structure human interaction. Second, it delineates distinguishing factors between the two types of metonymies. Those are the nature of the source and the (mis)match in the intentionality of producer and perceiver. Third, it makes an overview and provides real examples of what aspects of human interaction are metonymic. Its elements, including the content of the message, the identity, proxemics, and kinesics of the participants, and the context of the interaction, can be metonymic. Its processes, namely those of language production and reception, are as well inherently metonymic. Overall, the article shows that metonymy, understood as relatedness or association, pervades human interaction and plays an important role in its success.