Erik Erikson's psychosocial development theory outlines eight stages that individuals go through from infancy to late adulthood. Each stage presents a unique conflict that must be resolved for healthy psychological development. Successful resolution leads to a positive outcome, while failure can result in difficulties in later stages. The stages include trust vs. mistrust in infancy, autonomy vs. shame and doubt in early childhood, and identity vs. role confusion during adolescence. Erikson emphasized the importance of social relationships and cultural influences in shaping personality and behavior throughout a person's life.