Reactive oxygen species have been implicated in the pathogenesis of several human diseases. However, it is difficult to quantitate reactive oxygen metabolites because of their reactive nature and short half lives. Instead, analyses of secondary or end products produced by the attack of free radicals on lipids, proteins or other cellular components are preferred. These indirect methods usually give misleading results due to their poor specificity and sensitivity. Chemiluminescence (CL) is the production of light generated from chemical sources. It may be utilized as a direct noninvasive method for measuring reactive oxygen species. Different probes with different selectivities can be used in CL measurements. The use of CL measurements in quantitating free radical reactions, advantages over other methods and its applications on a tissue injury model will be presented.