Physical labels such as stable nitroxide radicals, luminescent and photochromic chromophores, so-called Mossbauer atoms and electron-dense assemblies of heavy atoms have proved to be effective tools in solving many problems at the molecular level in biological systems. These physical labels are used as 'molecular rulers' to measure the distances between chosen groups and to measure the size, form and microrelief of objects. By providing information about these factors, the label provides information that can help the scientist to understand the structure of membranes, nucleic acids, enzymes and proteins and how they function. This volume covers all aspects of this field: the theoretical bases, the experimental techniques, and it also shows how to interpret the resulting data. It also critically discusses some recent results obtained with these techniques and gives an analysis of likely developments in the future.