Parvalbumin is a small (Mr 12,000), acidic (pI 4-5), Ca2+-binding protein of the EF-hand superfamily, that is very important from several points of view. First of all, this protein takes part in Ca2+ regulation of activity of some types of muscle cells, neurons and some other types of cells. At the same time, the exact physiological role of parvalbumin in some of these cells is not clear enough now. Second, parvalbumin has two high affinity Ca2+ binding sites and for this reason it is frequently used as a simple model Ca2+ binding protein. It is convenient for studies of effects of interactions between two calcium binding sites and is very useful for studies of calcium binding effects on interactions of the protein with another proteins, peptides, membranes and low molecular weight organic compounds, which frequently have physiological significance. Third, parvalbumin forms several partially folded intermediate states, which have been studied by many researchers interested in protein folding and stability problems. It is very attractive for studies of the properties and structure of intermediate molten globule-like states. The present book summarizes our knowledge on physical, chemical and physiological properties of parvalbumins.