The persistence of the constructivist agenda within learning and developmental theory suggests that underneath the seemingly disparate views of Piagetian, socioculturalist, and ecological perspectives lie some unifying concepts that find their mutual expression in constructivism. The paper contends that semiotics provides the conceptual means to reveal these correspondences, and further, that semiotics, specifically Peirce's (1965; 1991) triadic relationship of thought, or triadicity, is able to serve as both a unifying and guiding conceptual tool for developmental theories because it addresses their most fundamental concern: how the quality of thought can change over time. The paper discusses the semiotic underpinnings of each of these three theoretical approaches: Piagetians and Neo-Piagetians, Vygotsky and the socioculturalists, and the ecological approach founded by James and Eleanor Gibson. Specifically, the paper discusses the equilibration and reflective abstraction within the Piagetian and Neo-Piagetian tradition, conceptual development in Vygotsky's work, and differentiation and integration in the perceptual learning theory of Eleanor Gibson. The paper concludes that all these theories serve the same idea: that students should constantly be encouraged to construct, discover, or perceive a multiplicity of meanings, and that educational practices should cater to this view. (Contains 41 references.) (EV)