Fatness and obesity are body characteristics which are both ascribed and achieved for adults. Socioeconomic status (SES) is the ranking of individuals within complex societies. In traditional societies a direct relationship between SES and fatness exists, while in modern societies there is an inverse association between SES and obesity for adult women but mixed patterns for other age/sex groups. A framework recognizing the difference between variables on the physical and social level of analysis needs to be used to examine the relationship between fatness (a physical variable) and SES (a social variable). Different mechanisms are involved in the causal pathways where SES influences obesity and obesity influences SES. SES influences obesity by education, income, and occupation causing variations in behaviors which change energy consumption, energy expenditure and metabolism. Obesity influences SES when the perception of obesity is interpreted through prejudiced beliefs, with subsequent stigmatization and discrimination limiting access to higher SES roles.