Application of chemiluminescence (CL) for study of free-radical reactions in human and animal cells and tissues is considered in this review. Historically, the study of intrinsic (ultraweak) luminescence gave place to the measurement of CL in the presence of chemical activators (CL probes) and physical activators (sensitizers) of luminescence, which made the method much more sensitive and specific. The methods of CL and EPR are direct methods of radical investigation, though the advantage of the CL method consists in the fact that CL intensity is directly proportional to a steady-state concentration of the radicals responsible for luminescence (first of all, lipid and oxygen radicals) irrespective the activity of these radicals. The mechanisms of CL reactions in the absence of activators and in the presence of luminol and lucigenin are considered. Examples of various applications of the CL method in medical, biological, and clinical investigations are given including those for estimation of the phagocytic activity of cells, antioxidant activity, determination of toxicity, and other purposes.