One of the possible uses of biomarkers in epidemiological research is as early-outcome measures to predict the occurrence of clinical disease and to elucidate the biological mechanism of pathogenesis. This use is conceptually less straightforward than the well established use of biomarkers to improve or extend exposure assessment or to study interindividual variations in disease susceptibility. In principle, this form of use could accelerate or otherwise facilitate etiological research. However, in practice, the recent review literature suggests that this mode of biomarker use, especially in cancer epidemiology, is the least clear-cut and the least well developed. The recurrent problem is identifying biomarkers that: (1) are on the causal pathway, (2) have a high probability of progression to clinical disease, and (3) account for all or most of the cases of the specified clinical outcome. Such biomarkers would be most useful if they conferred a long lead-time relative to clinical disease occurrence.