The conditions of chronic obesity and overweight status are risk factors for lower cognitive performance, cognitive decline, cognitive deficit, and dementia. But lower cognitive performance early in life itself may be a risk factor for an increase in body weight over time. With this in mind, we review important papers in the literature that advance our knowledge of relations between weight and cognitive functioning, with an emphasis on papers that illustrate methodological and theoretical issues of importance. We describe the evolution in research on weight and cognition with respect to two major features: (a) the move backward in time from the diagnosis of dementia to the pre-clinical period of dementia in order to better identify risk factors; and (b) the evolution of studies from an earlier emphasis on obesity-related cardiovascular risk factors as major mediators of relations between obesity and cognition to a more recent emphasis on metabolic variables, lifestyle variables, genotype, and other mechanisms that explain relations among weight change, obesity, and cognition. We conclude that: 1) a complete understanding of the causal links between weight and cognitive functioning requires a lifespan perspective; 2) practically speaking, lifespan research may need to amalgamate and integrate research at different segments of the lifespan until such time that we can include the entire life cycle within a single study of weight and cognition; and 3) we need more studies that examine reciprocal relations between weight and cognition, especially early in life.