This book introduces the use of XANES spectroscopy to study metal complexes, particularly those of cobalt. After introducing the principles of the technique, the book explores cobalt coordination complexes (such as Co(NH?)? ?) and cobalamines (vitamin B??), analyzing their XANES spectra. It compares the characteristics of these complexes and highlights the applications of XANES in the study of metal proteins and enzymes. Finally, the book discusses the future prospects of this technique for understanding the role of metals in biological and medical systems.