Biomarkers are defined as measurements or characteristics that are evaluated as indicators of normal biological or pathogenic processes, or responses to therapeutic interventions. Biomarkers have become increasingly important to physicians, medical product developers (both drugs and devices), and regulatory agencies, impacting treatment decisions as well as drug approvals in their ability to define both efficacy (therapeutic benefit) and safety (the likelihood and extent of undesirable off-target responses). This article focuses on imaging biomarkers as employed in efficacy assessments in oncology drug development.