Electron Paramagnetic Resonance (EPR) is a spectroscopic technique that detects and characterizes molecules with unpaired electrons (i.e., free radicals). Although it is closely related to nuclear magnetic resonance (NMR) spectroscopy, EPR is still under development as an imaging modality. Unlike other imaging modalities, EPR is able to take direct measurements of tissue oxygen concentration in a manner that is not dependent on complex biological processes such as ligand binding specificity or tracer metabolism. We describe the use of single-point imaging (SPI) in radiofrequency, Fourier-transform EPR. We present the techniques for performing oxygen imaging using EPR with phantoms and in vivo. We present results from a study in which the two mouse legs (one normal and the other with tumor) were imaged over time as the mouse breathed alternately air, carbogen (95% O 2, 5% CO 2), and then air again. The reconstructed images demonstrate that the SPI EPR imaging technique readily distinguishes between the normal and tumor legs and can track the changes in tissue oxygen concentration in response to percentage of oxygen in breathing gas.