Total internal reflection fluorescence (TIRF) is an optical effect particularly well-suited to the study of molecular and cellular phenomena at liquid/solid interfaces. Such interfaces are central to a wide range of biochemical and biophysical processes: binding to and triggering of cells by hormones, neurotransmitters, and antigens; blood coagulation at foreign surfaces; electron transport in the mitochondrial membrane; adherence and mobility of bacteria, algae, and cultured animal cells to surfaces; and possible enhancement of reaction rates with cell surface receptors upon nonspecific adsorption and surface diffusion of an agonist. Liquid/solid interfaces also have important medical and industrial applications: e.g. detection of serum antibodies by surface immobilized antigens; and the manufacture of biochemical products by surface-immobilized enzymes.