Determining the capability of Vygotsky's cultural-historical theory to fulfill key functions of educational theory (such as revealing the complexity of apparently simple events) has been hindered primarily by the following factors: (a) inaccurate information about a minor discussion, the zone of proximal development (ZPD), attracted attention early on and became identified as a major aspect of his theory; (b) the unavailability of accurate translations of his complete theory for several years; and (c) the lack of key information in popular discussions of Vygotsky's work on scientific (subject matter) concepts, a major factor in cognitive development. This article first describes the original misconception of the ZPD, current extrapolations, and discrepancies between Vygotsky's thinking and those views of the ZPD. Then the pivotal role of subject matter concepts in cognitive development, their relationship to logical thinking, and levels of thinking (pseudoconcepts, preconcepts, true conceptual thinking) are discussed. Implications for education include (a) rethinking classroom practices (accurately assessing the ZPD, collaborative learning in the classroom), (b) providing a sound rationale for reducing the proliferation of content requirements, and (c) refuting the practice of introducing abstract ideas and complex intellectual capabilities in the early grades. Issues in constructing curricula according to Vygotsky's perspective are also discussed.