In this article, the authors introduce a new theoretical framework for understanding intersensory development. Their approach is based upon insights gained from adults who experience synesthesia, in whom sensory stimuli induce extra cross-modal or intramodal percepts. Synesthesia appears to represent one way that typical developmental mechanisms can play out by magnifying connections present in early life that are pruned and/or inhibited during development but persist in muted form in all adults. As such, the study of synesthesia provides valuable insights into the nature of intersensory development. The authors review evidence on the perceptual reality and neural basis of synesthesia, then summarize developmental models and evidence that its underlying mechanisms are universal in adults. They illustrate how evidence for consistent sensory associations in adults leads to predictions about toddlers' perception and present 3 bodies of work that have confirmed those hypotheses. They end by describing novel hypotheses about intersensory development that arise from this framework. Such intersensory associations appear to reflect intrinsic sensory cortical organization that influences the development of perception and of language and that may constrain the learning of environmentally based associations. (Contains 1 footnote.)