This report was requested by the Agency for Healthcare Research and Quality (AHRQ), the National Institutes of Health (NIH) Office of Dietary Supplements, and several other NIH institutes. It is one of several reports focusing on the role of omega-3 fatty acids in the prevention or treatment of various diseases. This particular report focuses on the effects of omega-3 fatty acids on cancer, specifically tumor incidence, clinical outcomes after cancer treatment, and tumor behavior. Over the past 40 years, an increasing number of physiological functions have been attributed to omega-3 fatty acids, including movement of calcium and other substances into and out of cells, relaxation and contraction of muscles, and regulation of clotting and secretion of substances that include digestive enzymes and hormones. Omega-3 fatty acids also play a role in the control of fertility, cell division, and growth, suggesting they may protect against certain types of cancer or may alter the response to cancer treatment. The major dietary sources of omega-3 fatty acids in the U.S. population are fish, fish oil, vegetable oils (principally canola and soybean), walnuts, wheat germ, and some dietary supplements.