This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book describes spectroscopic techniques which have contributed so much to the understanding of the active sites: electron paramagnetic resonance, nuclear magnetic resonance, X-ray absorption, and resonance Raman. It illustrates the great effort which went into the determination of amino-acid sequences. The book presents absorption and circular dichroic spectra. The polypeptide chains show a great diversity in size from the smaller ones of the small blue proteins and the copper-thioneins to those of the gastropodan hemocyanins, which are among the largest in nature. Special precautions are needed with the longer polypeptide chains in order to avoid proteolytic cleavage during their isolation, as shown for molluscan hemocyanins and ceruloplasmin.