The applications of in vivo EPR to date have been impressive despite the relatively short period of its development and the unusual instrumental and physical problems which others have outlined earlier in this text. Since the “object” is a large, water-containing, dielectric substance which is generally difficult to tune in a conventional X-band system, it is truly remarkable and impressive that the results obtained thus far have been so successful. This chapter will outline the applications, including feasibility studies, of the in vivo electron paramagnetic resonance (EPR) and EPR-imaging (EPRI) techniques.