In the early 1970s fluidity became a central topic of membrane biology. The classical paper by Singer and Nicolson (1972) on The fluid mosaic model of the structure of cell membranes’ integrated current speculations and results by visualizing the plasma membrane as a fluid lipid bilayer decorated and intercalated with proteins. In this model the lateral movement of membrane lipids and proteins was expected to have profound effects—for instance, on the rate of chemical reactions (Adam and Delbriick, 1968) or the transmission of hormonal signals (Cuatrecasas, 1974). Experimentally, lateral mobility in membranes was first inferred from cell fusion studies (Frye and Edidin, 1970), from patching and capping phenomena of immunoreceptors on lymphocytes (Taylor et al, 1971), from freeze-fracture studies of membrane intercalated particles (da Silva, 1972) and from the mobility of spinlabels in artificial lipid bilayers (Trauble and Sackmann, 1972). The first measurement of diffusional movement in a cell membrane, however, was provided by Cone (1972), who applied time-resolved absorption measurements to follow the rotational movement of rhodopsin in amphibian photoreceptors. Rhodopsin was found to be highly mobile, which strongly supported the fluid mosaic model.