In recent decades, experimental and clinical studies have associated the development of obesity with the composition of the gut microbiota. Mechanisms potentially involved in the contribution of gut microbiota to body weight gain include changes in energy extraction from the diet and the modulation of lipid metabolism, endocrine functions, and the immune system. The host's specific genetic heritage, the type and amount of food intake, chronic inflammation, reduced body energy expenditure, and exposure to obesogenic pollutants are also potential contributing factors. The pathophysiological processes involved in the relationship between gut microbiota and obesity are not fully understood, and further studies are needed to establish whether differences in gut bacterial diversity between obese and normal body weight individuals are the cause or a consequence of obesity.