Adolescence marks the transition from childhood to adulthood, spanning ages 10-19. This period is characterized by rapid physical, cognitive, and psychosocial changes, including puberty, identity formation, and brain development. Key theories in adolescent development include Erikson's psychosocial theory, Piaget's cognitive development theory, and Bronfenbrenner's ecological systems theory. These frameworks help explain the complex interplay of biological, psychological, and social factors shaping adolescent growth.