Nutritional epidemiology examines the role of nutrition in the etiology of disease, monitors the nutritional status of populations, develops and evaluates interventions to achieve and maintain healthful eating patterns among populations, and examines the relationship and synergy between nutrition and physical activity in health and disease.Similar to the early studies of nutrition deficiency, associations between dietary factors and chronic disease may be observed long before a specific etiologic factor can be identified (Jacob, 1999). However, the path from observing to curing disease is more complicated for chronic diseases than deficiency diseases, because chronic disease etiology is multifactorial and diseases take many years to develop or manifest.Evidence from a variety of study designs is required to establish a definitive relationship between diet and disease. Basic biochemistry and physiology, cell culture experiments, laboratory animal studies, and human metabolic studies provide pertinent mechanistic data to implicate a role for a specific dietary factor in carcinogenesis. However, these studies cannot prove that a particular dietary factor will cause or prevent a cancer in humans. Proof can only be established in human studies, preferably through randomized intervention trials. However, such trials are not always feasible. One challenge is the high cost of long-term studies, given that cancer takes years to develop. In addition, it is ethically implausible to test the relationship between a potentially harmful exposure and cancer in humans. For these reasons, the bulk of the available evidence for a particular diet–cancer relationship is garnered from observational nutritional epidemiological studies. This chapter provides an overview of the nutritional epidemiology of cancer, with particular emphasis on study design, dietary assessment, and analysis, as well as interpretation of findings.