Calcium plays an important role in a wide variety of biological processes. This divalent metal ion can bind to a large number of proteins; by doing so it modifies their biological activity or their stability. Because of its distinct che- cal properties calcium is uniquely suited to act as an on–off switch or as a light dimmer of biological activities. The two books entitled Calcium-Binding Protein Protocols (Volumes I and II) focus on modern experimental analyses and methodologies for the study of calcium-binding proteins. Both extracel- lar and intracellular calcium-binding proteins are discussed in detail. H- ever, proteins involved in c