A longitudinal design is one that measures the characteristics of the same individuals on at least two, but ideally more, occasions over time. Its purpose is to address directly the study of individual change and variation. Longitudinal studies are expensive in terms of both time and money, but they provide many significant advantages relative to cross-sectional studies. Indeed, longitudinal studies are essential for understanding developmental and aging-related changes because they permit the direct assessment of within-person change over time and provide a basis for evaluating individual differences in level as separate from the rate and pattern of change as well as the treatment of selection effects related to attrition and population mortality.