Dietary assessment plays a crucial role in our ability to detect relationships between dietary exposure and disease causation. Nutritional problems are at the root of major mass diseases that are impediments to progress toward national and international health goals. This is true for chronic undernutrition and famine as well as many of the chronic diseases afflicting middle-aged and elderly people in industrialized and developing countries. High-quality dietary assessment provides a sound scientific foundation for the primary prevention of mass diseases, whereas inadequate assessment can produce false-negative results and result in apparent inconsistencies between cross-population and within-population findings for a particular disease. The critical role of dietary assessment in the elucidation of disease causation is discussed with regard to high blood pressure, heart disease, breast cancer, and several other major chronic diseases. Improved approaches to dietary assessment need to be made more widely known, not only among research scientists and health practitioners, but also among policymakers who require high-quality dietary data for establishing nutrition goals and making policy decisions.