The aim of this paper is to provide an overview on Electron Paramagnetic Resonance (EPR) spectroscopy [or equivalently Electron Spin Resonance (ESR)], indicating the areas of consolidated and potential applications in the pharmaceutical, biomedical and clinical fields, and the instrumental and conceptual developments that are involved. The unique capability of EPR to detect and characterise free radicals and other species with unpaired electrons has been successfully applied to measure both in vitro and in vivo free radical intermediates from drugs and oxidative processes, to study the free radical/antioxidant properties of drugs, to study drug-membrane interactions or to monitor pharmaceuticals radio sterilisation. Although EPR has been mainly devoted in the past to qualitative analysis, following development of software and high-sensitivity instrumentation, applications of the technique have been extended more recently to the quantitative analysis. The potential value of in vivo EPR includes: drug detection in body fluids, pharmacokinetic and metabolism studies, direct detection or measurement of free radicals (especially nitric oxide, the critical physiological mediator of vascular homeostasis and cardiac function). In addition, because EPR almost uniquely can make repeated and accurate measurements of pO2 in tissues, another interesting area in the pharmaceutical field is oximetry (now emerging also for clinical use), and some examples of its application in drug/formulation development, and in preclinical studies are reported.