This selective review by Timothy A. Salthouse of cognitive aging focuses on five major questions: which abilities are related to age, how many distinct influences contribute to age-related cognitive changes, whether age-related declines in mean performance are associated with increases in between-person variability, what discrepancies exist between cross-sectional and longitudinal age comparisons, and what methods can be used to identify the causes of age-related influences on cognition. Despite the lack of definitive answers, significant progress has been made in understanding the patterns of cognitive aging. The review highlights that crystallized abilities tend to increase until the 60s, while fluid abilities decline linearly from early adulthood. Similar age-related declines are observed in various neuropsychological tests. The number of distinct age-related influences is likely fewer than the number of variables exhibiting age relations, suggesting that researchers should consider general influences when interpreting age differences. Cross-sectional data often show large mean declines without increases in between-person variability, indicating a shift in the entire distribution rather than an increase in the breadth of the distribution. The discrepancy between cross-sectional and longitudinal age trends is partly due to practice effects in longitudinal data. Methodologically, correlation-based procedures and mediation analyses are used to investigate causal relations, but these methods have limitations. The review emphasizes the need for further research to fully understand the causes of age-related cognitive changes.This selective review by Timothy A. Salthouse of cognitive aging focuses on five major questions: which abilities are related to age, how many distinct influences contribute to age-related cognitive changes, whether age-related declines in mean performance are associated with increases in between-person variability, what discrepancies exist between cross-sectional and longitudinal age comparisons, and what methods can be used to identify the causes of age-related influences on cognition. Despite the lack of definitive answers, significant progress has been made in understanding the patterns of cognitive aging. The review highlights that crystallized abilities tend to increase until the 60s, while fluid abilities decline linearly from early adulthood. Similar age-related declines are observed in various neuropsychological tests. The number of distinct age-related influences is likely fewer than the number of variables exhibiting age relations, suggesting that researchers should consider general influences when interpreting age differences. Cross-sectional data often show large mean declines without increases in between-person variability, indicating a shift in the entire distribution rather than an increase in the breadth of the distribution. The discrepancy between cross-sectional and longitudinal age trends is partly due to practice effects in longitudinal data. Methodologically, correlation-based procedures and mediation analyses are used to investigate causal relations, but these methods have limitations. The review emphasizes the need for further research to fully understand the causes of age-related cognitive changes.