AbstractBiomineralization is the matrix-directed calcification of tissue in living organisms. The deposition of different polymorphs of calcium phosphate or calcium carbonate is a highly regulated process. It may involve cell-controlled mechanisms with vesicular delivery of inorganic material to the extracellular matrix and cell-independent processes mediated by dedicated matrix proteins. These proteins promote the formation of microscopic crystals of defined size and shape, which combine to form bio-inorganic materials with unique properties. Successful biomineralization is correlated with structural elements, such as matrix proteins involved in the nucleation process. Circular dichroism (CD) is a spectroscopic technique for the determination of a secondary structure of proteins and has therefore been applied for studying numerous biomineralization promoter proteins. This article reviews and compares CD data on matrix proteins from different contexts, such as eggs, seashells, and teeth. It highlights the potential of CD for secondary structure determination and quantification and points out pitfalls that may lead to misinterpretation of CD spectra. The data suggest that most biomineralization promoter proteins contain domains of different secondary structure with predominantly unordered conformation. However, they may acquire a higher degree of order initiated by environmental factors such as pH, presence of cations, or charged surfaces.