The aim of this work was the noninvasive in vivo oxygen measurement in tumors by electron paramagnetic resonance (EPR) spectroscopy. For EPR oximetry, stable oxygen sensitive radicals – so called spin probes – have to be employed. After comprehensive physicochemical analysis of different spin probes, various nano- and microparticulate formulations were developed for these radicals. The formulations ensured their biocompatibility and long term stability. The microparticles, which could be implanted simultaneously with the tumor cells, enabled a noninvasive and reproducible assessment of absolute oxygen levels in the tumor tissue. The accuracy was confirmed by comparison with well established reference methods. With these preclinical oxygen measurements, important information on the status and progress of tumors and other diseases can be gained, paving the way toward individualized and more effective therapy strategies.