The ready availability of large portions of inexpensive, palatable, calorie-dense foods is a characteristic of an obesogenic environment. Portion size has a robust effect on energy intake and, therefore, may play an important role in the etiology of obesity. This entry reviews trends in the size of portions available commercially, provides an overview of the effects of portion size on energy intake in adults and children, and presents suggestions for how dietary and food marketing strategies can be used to help get portions back in synchrony with consumers’ energy needs.