Chemiluminescence is a simple and reliable method of assessing phagocytic function. The bactericidal properties of phagocytes are dependent on the production of powerful oxidising species by the respiratory burst. These reactive oxygen radicals react with biological substrates to form excited compounds which then relax to their ground state by photon emission. This energy release is in the form of light which can be amplified by chemiluminescent probes and measured in a luminometer. Activation of cells is achieved using various agents that stimulate the respiratory burst. There is a close correlation between chemiluminescence and other methods of assessing phagocytic function, including bactericidal ability. The technique can be used to assess the function of polymorphonuclear leukocytes, monocytes, and tissue macrophages in response to disease, drugs, and toxins. This article describes the theory of cellular chemiluminescence, and the use of chemiluminescent probes and various cellular stimuli. Practical aspects of cell isolation and factors affecting chemiluminescence are considered. Finally, the clinical applications of chemiluminescence are discussed.