SummaryObesity is an important risk factor of death worldwide, with increasing rates in adults and children. Convincing evidences underline that body fatness is also a key risk factor for several types of tumors, in particular those of the gastrointestinal tract; among them, oesophageal, liver, pancreatic and colon rectal cancers show the strongest link with obesity. Unbalance in specific biological pathways (including adipokine secretion, insulin-like growth factor-1 pathway, redox state and inflammation), often accompanying obesity, contribute to cancer onset and progression. By acting on specific lifestyle and cancer risk factors, including food, nutrition, physical activity and body composition, cancer incidence may be effectively decreased. In this review, we summarize the most convincing evidence on the relationship between obesity and gastrointestinal tumors, underling the potential mechanisms of action, as well as strategies that may be useful for cancer prevention.