To ensure efficacy but avoid toxicity, many drugs require monitoring of their concentrations in the patient’s blood during therapy. Currently, therapeutic drug monitoring (TDM) is done in diagnostic laboratories that need trained personnel and expensive instruments. For this reason, the technique is only used when absolutely necessary and patients in regions with poor infrastructure cannot benefit from it at all. Even though methods that can be used at bedside or at home would have obvious advantages in terms of therapeutic outcome and convenience, none exist yet. This thesis introduces a new class of bioluminescent sensor proteins that could fill this need. LUCIDs (LUCiferase-based Indicators for Drugs) rely on the blue emission of a luciferase and the analyte-dependent resonance energy transfer to an orange fluorophore. LUCIDs provide a strong signal and an exceptional dynamic range, permitting measurements by spotting minimal sample volumes on paper and recording emission using a simple digital camera. We present the construction of such sensor proteins for the anticancer agent methotrexate, the antiepileptic topiramate, the immunosuppressants tacrolimus, sirolimus, and cyclosporin A, as well as the cardiac glycoside digoxin. Finally, we show that LUCIDs can precisely quantify drug concentrations in samples from hospital patients. This low-cost point-of-care method could make therapies safer, increase the convenience of doctors and patients, and make therapeutic drug monitoring available in regions with poor infrastructure.