This review outlines the association of obesity with risk of colorectal cancer and the potential underlying mechanisms from an epidemiological perspective. Current research indicates that there is a moderate but consistently reported association between general obesity (as determined by BMI) and colorectal cancer incidence and mortality. The relative risk associated with obesity is higher for cancer of the colon than for cancer of the rectum and it is higher in men than in women. By contrast, abdominal adiposity (as determined by waist circumference or waist-to-hip ratio) is similarly strongly associated with colon cancer in men and women, suggesting that abdominal adiposity is a more important risk factor for colon cancer than general adiposity, at least in women. Putative mechanisms that may account for the link between adiposity and colorectal cancer risk include hyperinsulinemia, insulin resistance, inflammation, altered immune response, oxidative stress, as well as disturbances in insulin-like growth factors, adipokines, and sex steroids. Understanding the link between obesity and colorectal cancer may pave the way for targeted prevention of colorectal cancer morbidity and mortality.