The phenomenon of age-related cognitive decline has long been controversial, both in terms of mere existence, and with respect to how it is explained. Some researchers have dismissed it as an artifact of declining health or lower levels of education, and others have attributed it to general changes occurring in the external environment. Still other interpretations have been based on the "use it or lose it" principle -- known as the Disuse Hypothesis -- or on the idea that there are qualitative differences in either the structure or the process of cognition across the adult years. Perhaps the most popular approach at present relies on the information-processing perspective and attempts to identify the critical processing component most responsible for age-related differences in cognition. The primary purposes of this book are first to review the evidence of age-related differences in cognitive functioning and then to evaluate the major explanations proposed to account for the negative relations between age and cognition that have been established. Included is a discussion of theoretical dimensions and levels of scientific theorizing assumed to be helpful in understanding and evaluating alternative perspectives on cognitive aging. The various perspectives are then covered in detail and analyzed. The text concludes with observations about the progress that has been made in explaining cognitive aging phenomena, plus recommendations for research practices that might contribute to greater progress in the future.