This thesis reports on the validity and the reproduciblity of methods assessing food consumption in groups of free-living Dutch adults consuming self-selected diets. The validity of a method, that is the demonstration that a method measures what it is intended to measure, can only be assessed by comparing it with an independent method of indisputable accuracy. There Is no such absolute method because of the nature of the data to be collected. Instead, it may be possible to establish either the relative validity or the concurrent validity. The relative validity evaluates the method in terms of another generally accepted method, designed to measure the same concept. The concurrent validity compares the results of a method with a biological marker. Validity studies refer to information bias and systematic response errors. A method is commonly called reliable or reproducible if it gives the same results when used repeatedly in the same situation. The problem in food consumption surveys is that the situation is never absolutely identical. Reproducibility refer to the biological within- person variation (a true day-to-day variation) as well as to random response errors, because these two sources of variation can hardly be separated. Many objectives can be served in conducting food consumption studies. It is essential to distinguish between food consumption surveys mainly used for national food planning and administration and surveys in which the emphasis is primarily on the relation between nutrition and health. Distinction is necessary because each objective demands a different type of information. In this thesis only methods assessing food consumption used in studies on the relationship between diet and health have been examined. Chapter 2 describes the state of the art of methods assessing food consumption. There are many sources of error in these types of studies as Table 4 in Chapter 2 clearly shows. For epidemiological studies, however, it is not always necessary to produce accurate results for each individual, ...