Although the definition is evolving, the biomarker can be defined broadly as an objectively measured biological characteristic that reflects normal biological processes, pathogenic processes or even pharmacologic responses to therapeutic interventions. Biomarkers can be used to estimate exposure to a risk factor. This method can reduce bias and improve validity in epidemiologic studies. Exposure to smoking can be ascertained by asking patients about their smoking history, but the effects of smoking also can be measured by changes in DNA (carcinogenic DNA adducts) or by the amount of serum and hair continine. Here, Glick examines why the ability to recognize, evaluate and understand the uses of existing and emerging biomarkers is an essential skill required of all biomedical health care professionals.