This chapter presents fluorescence methods for studying membrane dynamics. Fluorescence measurements can reveal structural and dynamic properties of animal cell plasma membranes that are crucial to their physiological function. The plasma membrane not only separates the cell interior from the surroundings, but also participates in many active processes. This include the reception and transmission of biochemical signals that controls metabolism, differentiation, and other cellular functions and transport of molecules along or through the membrane and the control of locomotion and morphology. The extent to which plane-polarized light is absorbed by a molecule depends on the extent to which one of its resonant transition dipoles is parallel to the plane of polarization. The light emitted as fluorescence is parallel to the emission transition dipole. The interpretation of fluorescence polarization measurements in molecular terms requires that transition moments, fixed relative to the three-dimensional coordinates of the molecule, relate to the plane of polarization of the incident excitation light, which is fixed in a laboratory frame of reference.