Expressive arts therapy is introduced in this exploration of the basic principles and foundations in using the arts in psychotherapy. A discussion of polyaesthetics, crystallization, intermodal theory, and the roles of expression and catharsis provide a theoretical foundation for understanding this creative approach to therapy. Asking the question "What happens when a client's art-making is received by the therapist?, the special considerations in the practice of expressive arts therapy are summarized, including relationship as art, play, structure and framing, low-skill and high-sensitivity work, and the phenomenon of the surprising experience that comes when a therapist and client work in a creative space together. This is a fundamental resource for all who consider the use of the arts in therapy.