The availability of large food portions facilitates overconsumption. Experimental studies consistently demonstrate that serving large amounts of foods and beverages leads individuals to consume more energy than they require. This portion size effect has been observed across a variety of different foods, is robust across different populations, and can be sustained over several months or more. Furthermore, there is evidence that prolonged provision of large portions can lead to weight gain. The robust nature of this effect has led to efforts to identify strategies to manage food and beverage portions. Certain characteristics of the consumer (e.g., satiety and food responsiveness) and the available food (e.g., palatability, value, and energy density) relate to the magnitude of the portion size effect, and these are potential targets for interventions to attenuate the response. For example, there is some evidence that making smaller meal portions of equal or better value than larger portions can promote portion control. To date, the most reliable and effective method to moderate energy intake from large portions is reducing the energy density of the diet. This can be achieved by increasing the proportions of fruits and vegetables relative to more energy-dense foods or ingredients, and this strategy has been successfully applied in the context of controlled and naturalistic settings. Future studies should build on current knowledge to better understand individual and contextual variability and diversity in the response to portion size. A more comprehensive understanding of the portion size effect can lead to the development of a systems approach in which the food, individual, and environment are targeted simultaneously to counter the effects of large portions.