In this monograph, we ask whether various kinds of intellectual, physical, and social activities produce cognitive enrichment effects-that is, whether they improve cognitive performance at different points of the adult life span, with a particular emphasis on old age. We begin with a theoretical framework that emphasizes the potential of behavior to influence levels of cognitive functioning. According to this framework, the undeniable presence of age-related decline in cognition does not invalidate the view that behavior can enhance cognitive functioning. Instead, the course of normal aging shapes a zone of possible functioning, which reflects person-specific endowments and age-related constraints. Individuals influence whether they function in the higher or lower ranges of this zone by engaging in or refraining from beneficial intellectual, physical, and social activities. From this point of view, the potential for positive change, or plasticity, is maintained in adult cognition. It is an argument that is supported by newer research in neuroscience showing neural plasticity in various aspects of central nervous system functioning, neurochemistry, and architecture. This view of human potential contrasts with static conceptions of cognition in old age, according to which decline in abilities is fixed and individuals cannot slow its course. Furthermore, any understanding of cognition as it occurs in everyday life must make a distinction between basic cognitive mechanisms and skills (such as working-memory capacity) and the functional use of cognition to achieve goals in specific situations. In practice, knowledge and expertise are critical for effective functioning, and the available evidence suggests that older adults effectively employ specific knowledge and expertise and can gain new knowledge when it is required. We conclude that, on balance, the available evidence favors the hypothesis that maintaining an intellectually engaged and physically active lifestyle promotes successful cognitive aging. First, cognitive-training studies have demonstrated that older adults can improve cognitive functioning when provided with intensive training in strategies that promote thinking and remembering. The early training literature suggested little transfer of function from specifically trained skills to new cognitive tasks; learning was highly specific to the cognitive processes targeted by training. Recently, however, a new generation of studies suggests that providing structured experience in situations demanding executive coordination of skills-such as complex video games, task-switching paradigms, and divided attention tasks-train strategic control over cognition that does show transfer to different task environments. These studies suggest that there is considerable reserve potential in older adults' cognition that can be enhanced through training. Second, a considerable number of studies indicate that maintaining a lifestyle that is intellectually stimulating predicts better maintenance of cognitive skills and is associated with a reduced risk of developing Alzheimer's disease in late life. Our review focuses on longitudinal evidence of a connection between an active lifestyle and enhanced cognition, because such evidence admits fewer rival explanations of observed effects (or lack of effects) than does cross-sectional evidence. The longitudinal evidence consistently shows that engaging in intellectually stimulating activities is associated with better cognitive functioning at later points in time. Other studies show that meaningful social engagement is also predictive of better maintenance of cognitive functioning in old age. These longitudinal findings are also open to important rival explanations, but overall, the available evidence suggests that activities can postpone decline, attenuate decline, or provide prosthetic benefit in the face of normative cognitive decline, while at the same time indicating that late-life cognitive changes can result in curtailment of activities. Given the complexity of the dynamic reciprocal relationships between stimulating activities and cognitive function in old age, additional research will be needed to address the extent to which observed effects validate a causal influence of an intellectually engaged lifestyle on cognition. Nevertheless, the hypothesis that an active lifestyle that requires cognitive effort has long-term benefits for older adults' cognition is at least consistent with the available data. Furthermore, new intervention research that involves multimodal interventions focusing on goal-directed action requiring cognition (such as reading to children) and social interaction will help to address whether an active lifestyle enhances cognitive function. Third, there is a parallel literature suggesting that physical activity, and aerobic exercise in particular, enhances older adults' cognitive function. Unlike the literature on an active lifestyle, there is already an impressive array of work with humans and animal populations showing that exercise interventions have substantial benefits for cognitive function, particularly for aspects of fluid intelligence and executive function. Recent neuroscience research on this topic indicates that exercise has substantial effects on brain morphology and function, representing a plausible brain substrate for the observed effects of aerobic exercise and other activities on cognition. Our review identifies a number of areas where additional research is needed to address critical questions. For example, there is considerable epidemiological evidence that stress and chronic psychological distress are negatively associated with changes in cognition. In contrast, less is known about how positive attributes, such as self-efficacy, a sense of control, and a sense of meaning in life, might contribute to preservation of cognitive function in old age. It is well known that certain personality characteristics such as conscientiousness predict adherence to an exercise regimen, but we do not know whether these attributes are also relevant to predicting maintenance of cognitive function or effective compensation for cognitive decline when it occurs. Likewise, more information is needed on the factors that encourage maintenance of an active lifestyle in old age in the face of elevated risk for physiological decline, mechanical wear and tear on the body, and incidence of diseases with disabling consequences, and whether efforts to maintain an active lifestyle are associated with successful aging, both in terms of cognitive function and psychological and emotional well-being. We also discuss briefly some interesting issues for society and public policy regarding cognitive-enrichment effects. For example, should efforts to enhance cognitive function be included as part of a general prevention model for enhancing health and vitality in old age? We also comment on the recent trend of business marketing interventions claimed to build brain power and prevent age-related cognitive decline, and the desirability of direct research evidence to back claims of effectiveness for specific products.