Recent advances in science have provided a variety of genetic, genomic, and protein-based methods to treat human diseases. These advances, and equally great strides in in vivo imaging and methods of tissue collection, have created an unprecedented opportunity to discover and develop biological markers of human disease. A biomarker is defined as a molecular, biological, or physical characteristic that indicates a specific physiologic state (see Table 1 for definitions). It is used in clinical practice to identify risk for disease, diagnose disease and its severity, guide intervention strategies, and monitor patient responses to therapy. When used in a clinical research setting, biomarkers may predict whether a drug or other intervention is safe and effective in a shorter time and at lower cost than clinical outcomes studies. For these and other reasons, including the ability to stratify patient groups based on objective criteria, biomarkers help promote regulatory approval of new therapeutic entities by pharmaceutical, biological, and device development teams.