In this paper we offer a theory of cross-modal objects. To begin, we discuss two kinds of linkages between vision and audition. The first is a duality. The the visual system detects and identifies surfaces ; the auditory system detects and identifies sources . Surfaces are illuminated by sources of light; sound is reflected off surfaces. However, the visual system discounts sources and the auditory system discounts surfaces. These and similar considerations lead to the Theory of Indispensable Attributes that states (...) the conditions for the formation of gestalts in the two modalities. The second linkage involves the formation of audiovisual objects, integrated cross-modal experiences. We describe research that reveals the role of cross-modal causality in the formation of such objects. These experiments use the canonical example of a causal link between vision and audition: a visible impact that causes a percussive sound. (shrink)