This book is the second volume in a series presenting new English translations of L.S. Vygotsky's writings on the holistic science of the child he called "pedology". It presents unique materials which reflect the development of Vygotsky's theoretical position at the last stage of his creative evolution in 1932-1934 and contributes to the number of original Vygotsky texts available in English. It includes the problem of age and age periodization; the structure and dynamics of age, psychological characteristics of age crises and diagnostics of development in relation to age, and the zone of proximal development, which became his most widely known but least understood theoretical innovation. This book places that concept in its context and makes it fully understandable for the first time. In addition, there are lectures and notes that Vygotsky made in preparation for lectures on six critical periods: birth, one year old, three, seven, and thirteen. Vygotsky also devotes chapters to thestable periods of infancy and early childhood and two whole chapters to school age. Future volumes in this series will explore Vygotsky's pedology of the adolescent.