What do we mean by theory in the arts, and what role does it play in that sense? Theory appears as an active, transformative, fluid, and communicative element of art practices, and research in the arts in particular. Theory is the fluid that perfuses and connects the territory of fine art and design, as well as other art forms such as music, architecture, writing, moving images, dance, and theatre. At the heart of this publication is the new field of research in theory and the way the ArtEZ Theory in the Arts research group investigates its possibilities and extends its boundaries. Editor Peter Sonderen currently chairs the professorship Theory in the Arts at ArtEZ University of the Arts.