This chapter explores theories of human development, also referred to as developmental psychology, as a knowledge base for professionals to integrate theory into case conceptualization in child psychotherapy. It provides a brief overview of the significant contributions of developmental psychology to the field of child psychotherapy that impact case conceptualization in the clinical treatment of children. Many theorists have shaped the study of human development, including Buford Jeanette Johnson, Anna Freud, Jean Piaget, Lev Vygotsky, Urie Bronfenbrenner, Erik Erikson, Jerome Kagan, John B. Watson, B. F. Skinner, Albert Bandura, Lawrence Kohlberg, Jerome Brunner, Robert J. Havighurst, and Emmy Werner. Collectively, their theories propose explanations of all aspects of human development, including psychosexual, cognitive, social, psychosocial, behavioral, and neurological development, along with memory, information processing theories, and resilience. The chapter includes educational theory in order to understand how children are challenged to learn not only internally, but also externally, as well.