Concept development was one of the areas that most centrally occupied Vygotsky's empirical research. In contemporary educational psychology, this topic is investigated in the field of conceptual change research. In most cases, concepts are seen as entities and structures that people have in their minds. In contrast, we exhibit in this chapter—consistent with Vygotsky's approach—conceptions as inherently cultural phenomena. The historical origin of the mentalistic treatment is examined and attributed to a dualistic (Saussurian) view of the sign. Against this position, we note that everything in the production of clinical interviews required for deriving conceptions is cultural through and through, including the talk and, therefore, the conceptions. To illustrate and empirically examine this point, we analyze a mystery transcript and show how a conversation concerning astronomy becomes intelligible to an anonymous analyst not because the participants are expressing in words what otherwise exists in their heads as concepts, but because they share a cultural and historical language that they also have in common with the analyst. Consistent with Vygotsky's turn towards a holistic view of the unity of culture, we discuss how conceptual language develops in the sense-giving fields that emerge in concrete social transactions.