It is generally believed but difficult to prove that diet plays a role in the risk of various diseases. This paper reviews strengths and deficiencies of select diet-assessment methods used in epidemiologic studies with particular reference to their use in the study of osteoporosis. Direct observation or weighed food records are useful primarily as validation for less intrusive methods. Complete food history by interview or food diary (by self report) is expensive and time consuming. A 24-h diet recall obtained by a trained dietitian can provide accurate, quantitative information on recent intake but does not represent usual intake. Food frequency questionnaires provide better estimates of usual diet but are less quantitative and subject to problems of recall and seasonality. No method is universally the best. Lack of an expected diet-disease association may reflect exposure misclassification, inadequate statistical power, or limited range of the nutrients studied. Given the differences in diet-assessment methods, studies of dietary calcium and osteoporotic fracture have had surprisingly similar results.