This monograph describes the theoretical bases and experimental prerequisites for methods such as spin fluorescence, triplet, Mössbauer, 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 of biological molecules. Each section concludes with a discussion of the results obtained from these techniques in connection with various problems of enzyme catalysis, electron transfer, molecular biophysics and molecular biology. The uses that labeling techniques can be put to for the investigation of whole cells and tissues are also discussed.