With the increase in the number of cancer patients worldwide in the coming years, the need for knowledge on the influence of lifestyle factors on cancer survival is increasing. In this paper, the current knowledge on diet, body mass index, and physical activity in relation to cancer outcome is summarized. Also, challenges are identified to be addressed to provide evidence-based recommendations for cancer patients. Studies in patients with breast, colorectal, or prostate cancer show that the influence of dietary factors on survival remains to be determined. Adiposity and a lack of physical activity, however, appear to influence cancer outcome negatively. Subsequent studies should focus on more detailed aspects of these lifestyle factors, such as body composition and dose of exercise, and on the timing with respect to diagnosis and treatment. Research should be directed toward investigating diet and physical activity simultaneously and on how changes in health behavior can be established.