The primary utility of parinaric acid as a fluorescent probe is its potential for detailed characterization of the probe environment at a fundamental level. This potential derives from the structural similarity of this species with normal membrane components and the known location and orientation of the probe. The applications of parinaric acid fall into four classes: detailed studies of model membrane structure and dynamics, studies of binding to lipophilic proteins, assays of enzymatic or exchange properties of lipids, and studies of biological membranes and other lipid-containing structures. The polarization of the absorption of parinaric acid is at least roughly aligned along the polyene chain. This has been determined by measurement of the polarization of the excitation of fluorescence of parinaric acid, oriented in urea inclusion crystals. The interpretation of fluorescence lifetime data for parinaric acid is complicated by the nonexponential behavior observed in even simple systems.