The primary objective of nutritional epidemiology is to identify, in combination with other forms of research, which aspects of diet and nutritional factors are causally related to cancer development. However, traditional epidemiology can evaluate with only a limited degree of specificity to which individual dietary factors an increased occurrence of cancer can be attributed. The two main reasons for this are: (1) dietary intake levels of specific foods or food constituents can be strongly intercorrelated; (2) dietary intake levels of specific food constituents are generally measured with rather large errors. Biochemical markers are increasingly seen as measurements that may help to overcome some of the above-mentioned methodological problems in nutritional epidemiology.