Biomarkers reflective of the molecular and genetic heterogeneity in colorectal cancers now guide certain aspects of clinical management and offer great potential for enrichment, stratification, and identification of novel therapeutic targets in drug development. Using case-based examples, this article reviews biomarkers that have an established role in the clinical management of colorectal cancer: mismatch repair protein testing and KRAS and BRAF mutational analysis. A selection of biomarkers undergoing validation for future clinical application is presented, and the dynamic and challenging interface between biomarkers in research and clinical practice is discussed.