The last 10 years have seen an enormous growth in our understanding of the molecular organisation of biological membranes. Experimental methods have been devised to meas­ ure the translational and rotational mobility of lipids and proteins, thereby furnishing a quantitative basis for the concept of membrane fluidity. Likewise, the asymmetry of bi­ layer membranes as evidenced by the asymmetric insertion of proteins and lipids has been put on firm experimental ground. At higher molecular resolution it has been possible to provide a detailed pi2ture of the molecular conformation and dynamics of lipids and, to some extent, even of small peptides embedded in a bilayer matrix. Many of these achieve­ ments would not have been possible without the application of modem spectroscopic methods. Since these techniques are scattered in a variety of specialized textbooks the present monograph attempts to describe the key spectroscopic methods employed in present-day membrane research at an intermediate level. There is no question that the elusive detailed structure of the biological membrane demands a multiplicity of experi­ mental approaches and that no single spectroscopic method can cover the full range of physical phenomena encountered in a membrane. Much confusion in the literature has arisen by undue generalizations without considering the frequency range or other limi­ tations of the methods employed. It is to be hoped that the present monograph with its comprehensive description of most modem spectroscopic techniques, will contribute to- .