This chapter contains sections titled: Free Radicals in Disease Processes Nature of Free Radicals Involved in Disease Processes and Potential Catalysts for Radical Formation Direct EPR Detection of Reactive Radicals In Vivo and Ex Vivo Spin Trapping of Reactive Radicals In Vivo and Ex Vivo Spin Scavenging of Reactive Radicals In Vivo and Ex Vivo Spin Trapping of Nitric Oxide Verification of the Occurrence of Radical-Mediated Processes Conclusions Acknowledgments References