Some studies suggest that people can maintain their cognitive abilities through "mental exercise." This has not been unequivocally proven. Retirement is associated with a large change in a person's daily routine and environment. In this paper, we propose two mechanisms how retirement may lead to cognitive decline. For many people retirement leads to a less stimulating daily environment. In addition, the prospect of retirement reduces the incentive to engage in mentally stimulating activities on the job. We investigate the effect of retirement on cognition empirically using cross-nationally comparable surveys of older persons in the United States, England, and 11 European countries in 2004. We find that early retirement has a significant negative impact on the cognitive ability of people in their early 60s that is both quantitatively important and causal. Identification is achieved using national pension policies as instruments for endogenous retirement.