 Piaget only read Vygotsky's work after his death.Nearing the end of his life, Vygotsky's later work involved adolescent development.However, his most important and widely known contribution is his theory for the development of "higher psychological functions," which considers human psychological development as emerging through unification of interpersonal connections and actions taken within a given socio-cultural environment (i.e., language, culture, society, and tool-use).