Clinical trials that include integral biomarkers to determine eligibility, assign treatment, or assess outcome must employ robust assays to measure the molecular analyte of interest. The decision to develop a biomarker assay into a test suitable for use in humans should be driven by clinical need, that is, there should be a clear clinical purpose for undertaking the test development. Supporting in vitro or in vivo research on the ability of the marker to distinguish subgroups of patients with a given characteristic is necessary. The magnitude of the difference in treatment effect expected with use of the marker should be sufficient to support differential treatment prescription for marker-positive and -negative patients. Analytical and clinical validation of the marker assay should be completed before the clinical trial is initiated to ensure that the assay is stable enough for clinical use throughout the trial. Clinical use of the assay requires that it be performed in a Clinical Laboratory Improvement Amendments-accredited laboratory, and the need to apply for an Investigational Device Exemption from the U.S. Food and Drug Administration should be considered. In this article we elaborate on the steps required to get a biomarker assay ready for use as an integral component of a clinical trial and give an example of the use of an integral assay in a phase III trial.