Child development refers to orderly changes that occur as children grow. It includes physical, cognitive, and social/emotional domains from conception through adolescence. Major 20th century theories include psychoanalytic theories of Freud and Erikson focusing on personality formation. Behavioral theories like behaviorism and social learning emphasized environmental influences. Biological theories highlighted innate development and critical periods. Cognitive theories from Piaget and Vygotsky described how children actively construct understanding. Bronfenbrenner's ecological systems theory viewed development as influenced by multiple environmental systems.