This paper considers "dynamic assessment" as a support for development. Dynamic assessment is based on the concept of Vygotsky's "zone of proximal development" and Feuerstein's theory, particularly his "structural cognitive modifiability theory". The author examines both theories, and "mediation" in dynamic assessment process. Vygotsky and Feuerstein viewed development as a dynamic process. Vygotsky clarified the processes of optionality and consciousness through teaching and learning in school education and play. Feuerstein doesn't use these terms, but his idea has common meanings.