Any behavior humans can perform, they can performed in an “ostensive” way, that is, overtly attracting attention and providing evidence of meaning. The communicative principle of relevance is a lawlike generalization about ostensive stimuli and hence about human communication. Here I present the principle as a pair of simple and general empirical statements: one on the audience side and one on the communicator side. I also summarize recent developments: Experimental tests have proved supportive, theoretical analyses have connected the principle to evolutionary and developmental perspectives, and the principle has been used more broadly than before, far beyond the traditional domains of semantics and pragmatics. These developments consolidate the communicative principle of relevance as fundamental to understanding human interaction.