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Baltes, Reese, and Nesselroade (1977) defined the task of developmental science as “ the description, explanation, and modification (optimization) of intraindividual change in behavior and interindividual differences in such change across the life span ” (p. 84, italics in original). This task was embraced by many over the last 100 years, and indeed the discipline has yielded a wealth of knowledge about the physical, cognitive, emotional, and social development of individuals across the life span. Developmental research has traditionally been conducted using one of two methodologies. One involves the repeated measurement of a sample of individuals, usually at the same age at the start of the study, over a period of time, termed a longitudinal study. The “task” in longitudinal studies is to find meaningful associations between age changes and changes in specific outcome behaviors or abilities of interest. The second involves the measurement of several samples of differing ages simultaneously, termed a cross-sectional study, in which the task is to discover age group differences in particular behaviors or abilities. This chapter reviews these two approaches from the vantage point of the general developmental model, discusses the advantages and pitfalls of each, and highlights exemplars of each from the developmental literature. Wohlwill (1970a) defines a variable as “developmental” ... log in or subscribe to read full text