The focus of this overview is on the use of EPR oximetry in biological systems with an emphasis on soluble probes. EPR oximetry measures oxygen concentrations based on changes in electron spin relaxation of probes such as trityl and nitroxide radicals. The experimental relaxation rates depend on the inherent relaxation of the probe, the effects of probe-probe collisions, and the relaxation enhancement caused by collisions of the probe with O2. To extract the oxygen concentration requires an understanding of all three contributions. The inherent relaxation rates of the probe depend on the rates of tumbling and on microwave frequency. The effects of probe-probe collisions depend on concentration, and for charged trityl radicals depend on ionic strength of the solution. The interactions of O2 with nitroxide and trityl radicals can be described in terms of exchange interactions and the dynamics of collisions in solution. The spatial volume elements that can typically be observed in EPR imaging experiments are large enough that the volume could contain a range of concentrations of radicals and of O2, which may result in distributions of linewidths and relaxation times that need to be considered in analyzing data. When using EPR oximetry in living systems, it is also necessary to consider whether the oxygen concentration is changing on the timescale of the experiment.