The incidence and prevalence of obesity and type 2 diabetes mellitus continues to rise worldwide, and there is evidence which suggests that both increased body mass index (i.e., obesity) and type 2 diabetes mellitus act as independent and synergistic risk factors for the development, mortality risk, and survival of certain cancers, and thereby contribute to the increasing incidence of cancers worldwide. Numerous pathways are hypothesised to account for the independent and combined effects of obesity and diabetes on cancer predisposition and outcomes. In understanding the individual and combined effects of obesity and diabetes on cancer predisposition and survival, there are confounding factors, such as medications, biases, and weight loss, to consider.