This monograph describes the theoretical bases and experimental prerequisites for methods such as spin fluorescence, triplet, Mossbauer, photochromic and electron-density labeling, including the procedures used to obtain specifically modified proteins, enzymes, biomembranes, nucleic acids, and other biological molecules. The fundamentals of the physical theory behind each technique is explained and details are given of how to interpret the experimental data obtained. Special sections deal with critical reviews of recent data on the structure, molecular dynamics and conformational transitions...