Innovation in basic and applied science has brought radiotracers to fruition as diagnostics. Non-invasive, longitudinal, and quantifiable molecular imaging is the key to diagnosing and monitoring numerous illnesses, with more to come from characterization of the clinical relevance of findings from genomics research. Radiotracers enable real-time in vivo studies of the effects of drug candidates on receptors, pathways, pharmacodynamics, and clinically relevant endpoints, thereby providing both early detection of pathophysiology to enable early intervention, and then monitoring of treatment responses to enable individualization of treatment regimens. We review developments which have translated imaging from bench to bedside, or biomarkers to diagnostics. Notable developments include (1) synthesis methods for rapid ¹¹C labeling of biomolecules to high specific radioactivity; (2) ligand-binding assays for screening molecular imaging agents rather than drugs; (3) in vivo imaging of radiotracers in animals; (4) discovering the imaging advantages of ^99mTc, ¹¹C, and ¹⁸F; (5) co-registration and automated quantitative assessment of high spatial resolution CT and MR images with molecular images from PET for longitudinal studies of treatment effect.