A bio-marker or biological marker is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated to examine normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention. Biomarkers are used in many scientific fields. Biomarkers used for medicine, or precision medicine, are a part of a relatively new clinical toolset. They are categorized in 3 primary ways according to their clinical applications. They are classified as molecular biomarkers, cellular biomarkers or imaging biomarkers. All 3 types of biomarkers have a clinical role in narrowing or guiding treatment decisions and follow a sub-categorization of being predictive, prognostic, or diagnostic. Predictive molecular, cellular, or imaging biomarkers that pass validation can serve as a method of predicting clinical outcomes. Predictive biomarkers are used to help optimize ideal treatments, and often indicate the likelihood of benefiting from a specific therapy. For example, molecular biomarkers situated at the interface of pathology-specific molecular process architecture and drug mechanism of action promise capturing aspects allowing assessment of an individual treatment response. This offers a dual approach to both seeing trends in retrospective studies and using biomarkers to predict outcomes. For example, in metastatic colorectal cancer predictive biomarkers can serve as a way of evaluating and improving patient survival rates and in the individual case by case scenario, they can serve as a way of sparing patients from needless toxicity that arises from cancer treatment plans.