Everybody knows that communication is important, but nobody knows how to define it. The best scholars refer to it. Free-speech law protects it. But no one-no scholar or judge-has successfully captured it. Few have even tried. This is the first article to define communication under the law. In it, I explain why some activities-music, abstract painting, and parading-are considered communicative under the First Amendment, while others-sex, drugs, and subliminal advertising-are not. I argue that the existing theories of communication, which hold that communicative behaviors are expressive or convey ideas, fail to explain what is going on in free-speech cases. Instead, communication hinges on the free will of the recipient. By this I mean that communication occurs when Person A conveys a thought to Person B, and Person B freely chooses whether to accept that thought. An act is communicative, in other words, if the important change that A wants to make in B’s mind occurs only if B wills it to, as happens during an argument. Reconceptualizing communication in this way-as behaviors meant to change minds through the free will of the listener-would solve deep and persistent First Amendment problems. It would explain which behaviors are communicative and therefore potentially covered by the First Amendment. Adopting the free-will theory would clarify the analysis in historically muddled areas such as the First Amendment treatment of nude dancing. But it would also shed light on the law governing new forms of behavior such as publication of computer-programming code. More broadly, the free-will theory of communication can point us in new directions. We are used to thinking of communication in ways that don’t describe it, and these errors may keep us from recognizing new forms of communication as they develop. Applying the free-will theory of communication, I argue, will prepare us for technological changes that will make our old metaphors for communication obsolete.