Cancer may not be completely the result of novel or inherited genetic mutations but may in fact be a largely preventable disease. Researchers have identified biochemicals, including n-3 (ω-3) fatty acids, tocopherols, β-sitosterol, and pedunculagin, that are found in walnuts and that have cancer-prevention properties. Mouse studies in which walnuts were added to the diet have shown the following compared with the control diet: (1) the walnut-containing diet inhibited the growth rate of human breast cancers implanted in nude mice by ∼80%; (2) the walnut-containing diet reduced the number of mammary gland tumors by ∼60% in a transgenic mouse model; (3) the reduction in mammary gland tumors was greater with whole walnuts than with a diet containing the same amount of n-3 fatty acids, supporting the idea that multiple components in walnuts additively or synergistically contribute to cancer suppression; and (4) walnuts slowed the growth of prostate, colon, and renal cancers by antiproliferative and antiangiogenic mechanisms. Cell studies have aided in the identification of the active components in walnuts and of their mechanisms of action. This review summarizes these studies and presents the notion that walnuts may be included as a cancer-preventive choice in a healthy diet.