This article presents an update of scientific evidence about the relation of obesity with the incidence or mortality by cancer. Almost all large epidemiological studies agree about the oncological risk of obesity, especially with regard to some types of cancers. Different physiopathological theories to explain this association have been proposed, from simple mechanical or dietetical causes to genetic or immunologic factors, including a possible role of hormones or adipocyte humoral factors. Moreover, obesity can play a role in the prognosis of certain cancers. Prevention of obesity must be included among the strategies to fight cancer.